Kalirin is a GDP/GTP exchange factor (GEF) for Rho proteins that modulates the actin cytoskeleton in neurons. Alternative splicing generates Δ-isoforms, which encode the RhoGEF domain, but lack the N-terminal Sec14p domain and first 4 spectrin-like repeats of the full-length isoforms. Splicing has functional consequences, with Kal7 but not ΔKal7 causing formation of dendritic spines. Cells lacking endogenous Kalirin were used to explore differences between these splice variants. Expression of ΔKal7 in this system induces extensive lamellipodial sheets, while expression of Kal7 induces formation of adherent compact, round cells with abundant cortical actin. Based on in vitro and cell-based assays, Kal7 and ΔKal7 are equally active GEFs, suggesting that other domains are involved in controlling cell morphology. Catalytically inactive Kal7 and a Kalirin fragment which includes only Sec14p and spectrin-like domains retain the ability to produce compact, round cells and fractionate as high molecular weight complexes. Separating the Sec14p domain from the spectrin-like repeats eliminates the ability of Kal7 to cause this response. The isolated Sec14p domain binds PIP 2 (3,5) and PIP3, but does not alter cell morphology. We conclude that the Sec14p and Nterminal spectrin-like domains of Kalirin play critical roles in distinguishing the actions of full-length and Δ-Kalirin.

Secretory granules carrying fluorescent cargo proteins are widely used to study granule biogenesis, maturation, and regulated exocytosis. We fused the soluble secretory protein peptidylglycine ␣-hydroxylating monooxygenase (PHM) to green fluorescent protein (GFP) to study granule formation. When expressed in AtT-20 or GH3 cells, the PHM-GFP fusion protein partitioned from endogenous hormone (adrenocorticotropic hormone, growth hormone) into separate secretory granule pools. Both exogenous and endogenous granule proteins were stored and released in response to secretagogue. Importantly, we found that segregation of content proteins is not an artifact of overexpression nor peculiar to GFP-tagged proteins. Neither luminal acidification nor cholesterol-rich membrane microdomains play essential roles in soluble content protein segregation. Our data suggest that intrinsic biophysical properties of cargo proteins govern their differential sorting, with segregation occurring during the process of granule maturation. Proteins that can self-aggregate are likely to partition into separate granules, which can accommodate only a few thousand copies of any content protein; proteins that lack tertiary structure are more likely to distribute homogeneously into secretory granules. Therefore, a simple "self-aggregation default" theory may explain the little acknowledged, but commonly observed, tendency for both naturally occurring and exogenous content proteins to segregate from each other into distinct secretory granules. INTRODUCTIONThe cellular life of secretory proteins begins with the cotranslational translocation of their nascent polypeptide chains into the lumen of the endoplasmic reticulum (ER). These proteins then proceed in the anterograde direction, through the Golgi complex and into the trans-Golgi network (TGN), which is considered a "sorting station" for secretory proteins (Farquhar and Palade, 1981). At the TGN, vesicles destined for different subcellular compartments bud while acquiring their cargo. In all cell types, a "constitutive secretory pathway" delivers soluble and membrane secretory proteins necessary for housekeeping functions to the appropriate compartment (e.g., plasma membrane or endosomes/ lysosomes; Turner and Arvan, 2000). Besides this constitutive pathway, specialized cell types such as endocrine, exocrine, neuroendocrine cells, and neurons possess a "regulated pathway" in which specialized organelles, the secretory granules, store proteins for release in response to an incoming signal (Arvan and Castle, 1998). The soluble proteins found in constitutive vesicles and regulated secretory granules differ, and the processes responsible for this segregation are the object of intensive investigation.Two models, "sorting for entry" and "sorting by retention," both having experimental support, have been proposed to explain how sorting of soluble content proteins occurs (Arvan and Castle, 1998). The sorting for entry hypothesis postulates the existence of one or more "membrane receptors" able to selectivel...

The vacuolar H+-ATPase (V-ATPase) establishes pH gradients along secretory and endocytic pathways. Progressive acidification is essential for proteolytic processing of prohormones and aggregation of soluble content proteins. The V-ATPase V0 subunit is thought to have a separate role in budding and fusion events. Prolonged treatment of professional secretory cells with selective V-ATPase inhibitors (bafilomycin A1, concanamycin A) was used to investigate its role in secretory-granule biogenesis. As expected, these inhibitors eliminated regulated secretion and blocked prohormone processing. Drug treatment caused the formation of large, mixed organelles, with components of immature granules and lysosomes and some markers of autophagy. Markers of the trans-Golgi network and earlier secretory pathway were unaffected. Ammonium chloride and methylamine treatment blocked acidification to a similar extent as the V-ATPase inhibitors without producing mixed organelles. Newly synthesized granule content proteins appeared in mixed organelles, whereas mature secretory granules were spared. Following concanamycin treatment, selected membrane proteins enter tubulovesicular structures budding into the interior of mixed organelles. shRNA-mediated knockdown of the proteolipid subunit of V0 also caused vesiculation of immature granules. Thus, V-ATPase has a role in protein sorting in immature granules that is distinct from its role in acidification.

The molecular mechanisms involved in the maturation of secretory granules, organelles that store hormones and neuropeptides, are poorly understood. As granule content proteins are processed, the composition of granule membranes changes, yielding constitutive-like secretion of immature content proteins and producing secretagogue-responsive mature granules. Constitutive-like secretion was not previously recognized as a process subject to regulation. We show that Kalirin and Trio, homologous Rho guanine nucleotide exchange factors (GEFs), which interact with a secretory granule resident protein, modulate cargo secretion from immature granules. Some of the Kalirin and Trio isoforms expressed in neuroendocrine cells colocalize with immature granules. Overexpression of their N-terminal GEF domain (GEF1) enhances secretion from immature granules, depleting cells of secretory cargo in the absence of secretagogue. This response requires GEF1 activity and is mimicked by Kalirin/Trio substrates Rac1 and RhoG. Accordingly, selective pharmacological inhibition of endogenous GEF1 activity decreases secretagogue-independent release of hormone precursors, accumulating product peptide in mature secretory granules. Kalirin/Trio modulation of cargo secretion from immature granules provides secretory cells with an extra layer of control over the sets of peptides released. Control of this step enhances the range of physiological responses that can be elicited, whereas lack of control could have pathological consequences.

The field of organellar proteomics has emerged as an attempt to minimize the complexity of the proteomics data obtained from whole cell and tissue extracts while maximizing the resolution on the protein composition of a single subcellular compartment. Standard methods involve lengthy density-based gradient and/or immunoaffinity purification steps followed by extraction, onedimensional or two-dimensional gel electrophoresis, gel staining, in-gel tryptic digestion and protein identification by mass spectrometry. In this paper, we present an alternate approach to purify subcellular organelles containing a fluorescent reporter molecule. The gel-free procedure involves fluorescence-assisted sorting of the secretory granules followed by gentle extraction in a buffer compatible with tryptic digestion and mass-spectrometry. Once the subcellular organelle labeled, this procedure can be done in a single day, requires no major modification to any instrumentation and can be readily adapted to the study of other organelles. When applied to corticotrope secretory granules, it led to a much enriched granular fraction from which numerous proteins could be identified through mass spectrometry.