Elastic fibres are an important component of the extracellular matrix and are made of two major components: the more abundant cross-linked elastic protein elastin and the multi-component microfibrils. The biosynthesis of elastic fibres is a complex process involving the interplay of many diverse proteins and genes with elastin as the major component. Tropoelastin is the soluble precursor of elastin and as such it plays a dominant role in elastogenesis. The expression of tropoelastin is under a complex control mechanism, with many isoforms existing. Numerous other components, including the microfibrillar proteins, the elastin-binding protein and lysyl oxidase, the enzyme which initiates elastin cross-linking, are involved in elastogenesis. Tropoelastin undergoes self-association under physiological conditions in a process referred to as coacervation, and this is thought to be a vital process during elastic fibre formation and in providing elasticity. Although various models explaining the elasticity of elastin have been put forward, only the fibrillar model is based on the coacervation ability of tropoelastin. With the molecular cloning of a number of components of the elastic fibre, the availability of these components is increasing and paves the way for in vitro modelling of complex interactions of the elastic fibre. This review emphasises the biochemistry of tropoelastin and its role in elastic fibre structure and assembly.

Coacervation of soluble tropoelastin molecules is characterized by thermodynamically reversible association as temperature is increased under appropriately juxtaposed ionic conditions, protein concentration and pH. Coacervation plays a critical role in the assembly of these elastin precursors in elastic fiber formation. To examine the effect of physiological parameters on the ability of tropoelastin molecules to associate, solutions of recombinant human tropoelastin were monitored spectrophotometrically by light scattering over a broad range of temperatures. Coacervation of recombinant human tropoelastin is strongly influenced by the concentration of protein and NaCl and to a lesser extent on pH. Trends towards maximal association are apparent when each of these parameters is varied. Remarkably, optimal coacervation is found at 37"C, 150 mM NaCl and pH 7-8. Using the data generated by time courses, estimates of thermodynamic parameters were made. These estimates confirm that coacervation is endothermic and is marked by a strong entropic contribution. Circular dichroism of recombinant human tropoelastin revealed that, rather than being random, the structure is compatible with being largely that, of an all-P protein (with secondary structure estimated to be 3% a-helix, 41 % P-sheet, 21 % jj-turn and 33 % other), exhibiting a spectrum as previously seen for tropoelastin populations and soluble elastin from naturally-derived sources.

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Following cellular secretion into the extracellular matrix, tropoelastin is transported, deposited, and crosslinked to make elastin. Assembly by coacervation was examined for an isoform of tropoelastin that lacks the hydrophilic domain encoded by exon 26A. It is equivalent to a naturally secreted form of tropoelastin and shows similar coacervation performance to its partner containing 26A, thereby generalizing the concept that splice form variants are able to coacervate under comparable conditions. This is optimal under physiological conditions of temperature, salt concentration, and pH. The proteins were examined for their ability to interact with extracellular matrix glycosaminoglycans. These negatively charged molecules interacted with positively charged lysine residues and promoted coacervation of tropoelastin in a temperature-and concentrationdependent manner. A testable model for elastin-glycosaminoglycan interactions is proposed, where tropoelastin deposition during elastogenesis is encouraged by local exposure to matrix glycosaminoglycans. Unmodified proteins are retained at ϳ3 M dissociation constant. Following lysyl oxidase modification of tropoelastin lysine residues, they are released from glycosaminoglycan interactions, thereby permitting those residues to contribute to elastin cross-links.

S U M M A R Y Four distinct genes encode tropomyosin (Tm) proteins, integral components of the actin microfilament system. In non-muscle cells, over 40 Tm isoforms are derived using alternative splicing. Distinct populations of actin filaments characterized by the composition of these Tm isoforms are found differentially sorted within cells (Gunning et al. 1998b). We hypothesized that these distinct intracellular compartments defined by the association of Tm isoforms may allow for independent regulation of microfilament function. Consequently, to understand the molecular mechanisms that give rise to these different microfilaments and their regulation, a cohort of fully characterized isoform-specific Tm antibodies was required. The characterization protocol initially involved testing the specificity of the antibodies on bacterially produced Tm proteins. We then confirmed that these Tm antibodies can be used to probe the expression and subcellular localization of different Tm isoforms by Western blot analysis, immunofluorescence staining of cells in culture, and immunohistochemistry of paraffin wax-embedded mouse tissues. These Tm antibodies, therefore, have the capacity to monitor specific actin filament populations in a range of experimental systems.

The temperature-dependent association of tropoelastin molecules through coacervation is an essential step in their assembly leading to elastogenesis. The relative contributions of C-terminal hydrophobic domains in coacervation were assessed. Truncated tropoelastins were constructed with N termini positioned variably downstream of domain 25. The purified proteins were assessed for their ability to coacervate. Disruption to domain 26 had a substantial effect and abolished coacervation. Circular dichroism spectroscopy of an isolated peptide comprising domain 26 showed that it undergoes a structural transition to a state of increased order with increasing temperature. Protease mapping demonstrated that domain 26 is flanked by surface sites and is likely to be in an exposed position on the surface of the tropoelastin molecule. These results suggest that the hydrophobic domain 26 is positioned to play a dominant role in the intermolecular interactions that occur during coacervation.

Tropomyosin (Tm) is a key component of the actin cytoskeleton and >40 isoforms have been described in mammals. In addition to the isoforms in the sarcomere, we now report the existence of two nonsarcomeric (NS) isoforms in skeletal muscle. These isoforms are excluded from the thin filament of the sarcomere and are localized to a novel Z-line adjacent structure. Immunostained cross sections indicate that one Tm defines a Z-line adjacent structure common to all myofibers, whereas the second Tm defines a spatially distinct structure unique to muscles that undergo chronic or repetitive contractions. When a Tm (Tm3) that is normally absent from muscle was expressed in mice it became associated with the Z-line adjacent structure. These mice display a muscular dystrophy and ragged-red fiber phenotype, suggestive of disruption of the membrane-associated cytoskeletal network. Our findings raise the possibility that mutations in these tropomyosin and these structures may underpin these types of myopathies.

A growing body of evidence suggests that the Golgi complex contains an actin-based filament system. We have previously reported that one or more isoforms from the tropomyosin gene Tm5NM (also known as ␥-Tm), but not from either the ␣-or ␤-Tm genes, are associated with Golgi-derived vesicles (Heimann et al., (1999). J. Biol. Chem. 274, 10743-10750). We now show that Tm5NM-2 is sorted specifically to the Golgi complex, whereas Tm5NM-1, which differs by a single alternatively spliced internal exon, is incorporated into stress fibers. Tm5NM-2 is localized to the Golgi complex consistently throughout the G1 phase of the cell cycle and it associates with Golgi membranes in a brefeldin A-sensitive and cytochalasin D-resistant manner. An actin antibody, which preferentially reacts with the ends of microfilaments, newly reveals a population of short actin filaments associated with the Golgi complex and particularly with Golgi-derived vesicles. Tm5NM-2 is also found on these short microfilaments. We conclude that an alternative splice choice can restrict the sorting of a tropomyosin isoform to short actin filaments associated with Golgi-derived vesicles. Our evidence points to a role for these Golgi-associated microfilaments in vesicle budding at the level of the Golgi complex. INTRODUCTIONThe actin microfilament system performs a broad range of cellular functions from regulating cell structure to cell motility and cytokinesis. The ability of microfilaments to independently perform such a broad array of functions may be facilitated by the sorting of isoforms of the primary components of microfilaments to different intracellular compartments. Actin, which provides the core microfilament polymer, is encoded by two isoforms in mammalian nonmuscle cells (Herman, 1993). Many microfilaments contain tropomyosin (Tm), a coiled coil protein that binds along the side of actin filaments (Phillips et al., 1979). There are at least 40 different isoforms of tropomyosin (Lees-Miller and Helfman, 1991;Dufour et al., 1998). Thus the potential for creation of microfilaments with unique actin and tropomyosin isoform composition is very extensive.Studies in a variety of systems have provided consistent evidence for sorting of actin and tropomyosin isoforms to different intracellular locations (reviewed in Lin et al., 1997;Gunning et al., 1998aGunning et al., , 1998b. Isoform sorting, coupled to different functional properties of actin and tropomyosins, provide an attractive approach for spatially specializing microfilament function (Gunning et al., 1998a). Nonmuscle tropomyosin isoforms protect actin filaments from severing (Burgess et al., 1987;Ishikawa et al., 1989). Tropomyosins regulate actin filament dynamics by affecting the activity of ADF/cofilin and the Arp 2/3 complex (Bamburg, 1999;Blanchoin et al., 2001;Ono and Ono, 2002) in an isoform specific manner (Bryce et al., 2003). They can also regulate actin filament organization by competing for binding with actin bundling proteins (Ishikawa et al., 1994), controlling myosin motor acti...