Three cDNA clones of 1.6 (3u), 1.2 (5g) and 0.6 (5b) kbp with probes covering the 3' end of the two unexpected regions show that three distinct mRNAs correspond to the three cDNAs. Moreover, three peripherin products, two minor 61 and 56 kd products in addition to the major 58 kd peripherin, are observed when poly(A)+ RNA is in vitro translated, the 61 kd peripherin being translated from the 3u-selected RNA. The three RNAs originate from alternative splicing of a unique peripherin gene, thus generating polymorphism of peripherin.

Three distinct mRNAs have been shown to be produced by alternative splicing from the unique mouse peripherin gene. They generate three translation products, one major form, Pe-58, and two minor forms, Pe-56 which possess a shorter C-terminal sequence, and Pe-61 in which an additional sequence has been inserted in the central rod domain (Landon et al., 1989, EMBO J. 8, 1719-1726). In this study, the simultaneous occurrence of multiple transcripts in murine nervous tissues and neuroblastoma cell lines was shown by PCR amplification of fragments overlapping the sites of alternative splicing. Recombinant peripherin isoforms were purified from E. coli expressing full-length cDNAs. Rabbit antisera were raised against synthetic peptides mimicking parts of the two C-terminal sequences and of the inserted sequence of Pe-61 and were immunoadsorbed until they became monoreactive. By western blot analysis, the peripherin isoforms were localised in neuroblastoma NB2a cell lysates and detergent insoluble fractions separated by two-dimensional electrophoresis. In addition, each isoform was resolved into several charge variants. At the cellular level, each antibody decorated the filament array of the NB2a cells, suggesting the participation of the minor peripherin isoforms in the intermediate filament network.

Intermediate filament proteins of the rat insulinoma RIN5F cell line were characterized. Two-dimensional gel analysis followed by immunostaining of proteins demonstrated that these cells express both peripherin and the low-molecular-mass neurofilament protein (NF-L); this was confirmed for peripherin by immunohistochemistry, peptide analysis and Northern blot. No expression of these proteins could be detected with these same methods either in the adult pancreas or in the tumor at the origin of the cell line, although such expression was apparent on sections of rat pancreas at embryonal day 16. These results were compared to those obtained on the rat pheochromocytoma PC12 cell line: expression in the adrenal medulla of the embryo, no expression either in the adult tissue or in the tumor, but solely in the derived cell line. The expression of neuronal intermediate filament proteins in the rat insulinoma RIN5F cell line is discussed in relation to its similarity in the rat pheochromocytoma PC12 cell line, and its meaning as to the developmental cell lineage; an ectodermal origin is suggested for the pancreatic islet cells.

The structural and functional properties of the uu (2 x 97 kDa) and cc (2 x 94 kDa) isoforms of platelet a-actinin have been compared. Structural differences between aa and cc are revealed by their peptide maps, obtained from limited proteolysis, and by their immunological cross-reactivity. Both isoforms stimulate the Mg ATPase activity of actomyosin, bind to F-actin (high-speed sedimentation) and cross-link or gel actin filaments (low-speed sedimentation and viscometry), in a calcium-dependent manner. The study of the interaction with Factin indicates that the binding of 1 molecule of UN or cc a-actinin/9 -11 actin monomers is sufficient to produce maximal gelation in the presence of EGTA. CaCI, at 0.1 mM strongly inhibits the binding of U U to F-actin and weakly that of cc, while it inhibits similarly the cross-linking of either uu or cc. The cross-linking efficiency of cc is 9, 7, 1.7 and 1.3 times higher than that of uu at 4, 20, 30 and 3 7 T , respectively. The hh form (2 x 96 kDa), which is a proteolytic product of uu [Y. Gache et al. (1984) Biochem. Biophys. Rcs. Commun. 124, 877-8811, behaves roughly as aa, but the calcium sensitivity of its binding to F-actin is intermediate between that of cia and cc. These results suggest that the effect of Ca2' concentration on the binding of platelet a-actinin to F-actin may be partly dissociated from the effect on the cross-linking. Human platelet a-actinin has been characterized as a nonmuscle a-actinin [12, 131. It has been found to exist as multiple isoforms, which differ in chain length [14], iinmunological cross-reactivity and sensitivity to a Ca2 '-dependent protease [15]. In this report we describe a comparative study of the effect on F-actin of the two homodimeric isoforms of platelet a-actinin, aa and cc, as well as of the bb form, which is a proteolytic product ofaa. The uu and cc isoforms differ in the calcium sensitivity of their binding to F-actin and in their efficiency to cross-link actin filaments. MATERIALS AND METHODSIsolution of a-uctinin a-Actinin was purified from human blood platelets as previously described [13]. However, the susceptibility of the uu isoform to calcium-dependent degradation [14, 151 led us to slightly modify the isolation procedure: shortening of the extraction time by ultrasonic thawing of frozen platelets, presence of 5 mM instead of 1 mM EGTA in the extraction and dialysis buffers and of 1 mM EGTA in the DEAE chromatography elution buffers, addition of leupeptin (1 pg/ ml) to the sample applied to the hydroxyapatite column and to the eluting buffers. With these modifications, the fractions FI, FII, and FIII of the hydroxyapatite elution contained more (I subunit (Fig. 1 A)

The gene encoding mouse peripherin, a neuronal intermediate filament protein, has been cloned. Its sequence, through 1021 nucleotides composing the 5'-flanking region, nine exons, eight introns and 547 nucleotides of the 3'-flanking region, as well as its transcription initiation site have been determined. The amino acid coding sequence differs from that of the rat peripherin gene. The mouse gene has an additional histidine near the N-terminal end, and shows three conservative and two non-conservative changes. The promoter sequence, containing the binding sites for transcription factors as well as other sequences is homologous to promoter regions of other type III intermediate filament protein genes and other neuronal-specific genes.

Human blood platelet actin was purified using 30% sucrose to extract actomyosin and potassium iodide to dissociate actomyosin and to depolymerize actin. Pure actin thus obtained resembles skeletic muscle actin in its polymerization properties, CD spectra and ability to activate myosin Mg2+‐ATPase. Isoelectric focusing gel analysis shows that human blood platelet actin exists in β and γ forms. The ratio of β to γ forms is of 5 in purified actin, in whole cell extract and in all the fractions studied.

Using a mouse cDNA probe encoding for the major part of peripherin, a type III intermediate filament protein, we have assigned, by in situ hybridization, the mouse and human peripherin genes, Prph, to the E-F region of chromosome 15 and to the ql2-ql3 region of chromosome 12, respectively. These regions are known as homologous chromosomal segments containing other intermediate filament genes (keratins) and also other genes which could be co-ordinately regulated.

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