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Crevenna et al. examine the mechanism by which secretory cargoes are segregated at the trans-Golgi network (TGN) for release into the extracellular space. The authors demonstrate that Ca2+-dependent changes in Cab45 oligomerization mediate sorting of specific cargo molecules at the TGN., Sorting and export of transmembrane cargoes and lysosomal hydrolases at the trans-Golgi network (TGN) are well understood. However, elucidation of the mechanism by which secretory cargoes are segregated for their release into the extracellular space remains a challenge. We have previously demonstrated that, in a reaction that requires Ca2+, the soluble TGN-resident protein Cab45 is necessary for the sorting of secretory cargoes at the TGN. Here, we report that Cab45 reversibly assembles into oligomers in the presence of Ca2+. These Cab45 oligomers specifically bind secretory proteins, such as COMP and LyzC, in a Ca2+-dependent manner in vitro. In intact cells, mutation of the Ca2+-binding sites in Cab45 impairs oligomerization, as well as COMP and LyzC sorting. Superresolution microscopy revealed that Cab45 colocalizes with secretory proteins and the TGN Ca2+ pump (SPCA1) in specific TGN microdomains. These findings reveal that Ca2+-dependent changes in Cab45 mediate sorting of specific cargo molecules at the TGN.

Sorting of proteins for secretion from cells is crucial for normal physiology and the regulation of key cellular events. Although the sorting of lysosomal hydrolases at the trans-Golgi network (TGN) for delivery to pre-lysosomes is well characterized, the corresponding mechanism by which secreted proteins are sorted for plasma-membrane delivery remains poorly understood. Recent discoveries have revealed a novel sorting mechanism that requires the linkage between the cytoplasmic actin cytoskeleton to the membrane-anchored Ca(2+) ATPase, SPCA1 (secretory pathway calcium ATPase 1), and the luminal 45 kDa Ca(2+)-binding protein, Cab45, for successful sorting of a subset of proteins at the TGN. We review progress in understanding these processes.

The cofilin CFL-1 recruits actin to the P-type calcium ATPase SPCA1 at the trans-Golgi network, thereby activating the ATPase, promoting Ca2+ influx, and driving secretory cargo sorting., The actin filament severing protein cofilin-1 (CFL-1) is required for actin and P-type ATPase secretory pathway calcium ATPase (SPCA)-dependent sorting of secretory proteins at the trans-Golgi network (TGN). How these proteins interact and activate the pump to facilitate cargo sorting, however, is not known. We used purified proteins to assess interaction of the cytoplasmic domains of SPCA1 with actin and CFL-1. A 132–amino acid portion of the SPCA1 phosphorylation domain (P-domain) interacted with actin in a CFL-1–dependent manner. This domain, coupled to nickel nitrilotriacetic acid (Ni-NTA) agarose beads, specifically recruited F-actin in the presence of CFL-1 and, when expressed in HeLa cells, inhibited Ca2+ entry into the TGN and secretory cargo sorting. Mutagenesis of four amino acids in SPCA1 that represent the CFL-1 binding site also affected Ca2+ import into the TGN and secretory cargo sorting. Altogether, our findings reveal the mechanism of CFL-1–dependent recruitment of actin to SPCA1 and the significance of this interaction for Ca2+ influx and secretory cargo sorting.

Protein kinase D (PKD) is known to be involved in the fission of transport carriers at the Golgi complex. This study demonstrates that PKD is important for the cleavage of interstack Golgi connections in G2 of the cell cycle and thus entry of cells into mitosis., Before entering mitosis, the stacks of the Golgi cisternae are separated from each other, and inhibiting this process delays entry of mammalian cells into mitosis. Protein kinase D (PKD) is known to be involved in Golgi-to–cell surface transport by controlling the biogenesis of specific transport carriers. Here we show that depletion of PKD1 and PKD2 proteins from HeLa cells by small interfering RNA leads to the accumulation of cells in the G2 phase of the cell cycle and prevents cells from entering mitosis. We further provide evidence that inhibition of PKD blocks mitotic Raf-1 and mitogen-activated protein kinase kinase (MEK) activation, and, as a consequence, mitotic Golgi fragmentation, which could be rescued by expression of active MEK1. Finally, Golgi fluorescence recovery after photobleaching analyses demonstrate that PKD is crucial for the cleavage of the noncompact zones of Golgi membranes in G2 phase. Our findings suggest that PKD controls interstack Golgi connections in a Raf-1/MEK1–dependent manner, a process required for entry of the cells into mitosis.

Skeletal muscle responds to exercise by activation of signalling pathways that co-ordinate gene expression to sustain muscle performance. MEF2 (myocyte enhancer factor 2)-dependent transcriptional activation of MHC (myosin heavy chain) genes promotes the transformation from fast-twitch into slow-twitch fibres, with MEF2 activity being tightly regulated by interaction with class IIa HDACs (histone deacetylases). PKD (protein kinase D) is known to directly phosphorylate skeletal muscle class IIa HDACs, mediating their nuclear export and thus derepression of MEF2. In the present study, we report the generation of transgenic mice with inducible conditional expression of a dominant-negative PKD1kd (kinase-dead PKD1) protein in skeletal muscle to assess the role of PKD in muscle function. In control mice, long-term voluntary running experiments resulted in a switch from type IIb+IId/x to type IIa plantaris muscle fibres as measured by indirect immunofluorescence of MHCs isoforms. In mice expressing PKD1kd, this fibre type switch was significantly impaired. These mice exhibited altered muscle fibre composition and decreased running performance compared with control mice. Our findings thus indicate that PKD activity is essential for exercise-induced MEF2-dependent skeletal muscle remodelling in vivo.

Pheochromocytoma (PCC) is a usually benign tumor originated in the majority of patients from the adrenal medulla. Regarding sporadic forms of PCC, mechanisms of pathogenesis are largely unknown. Recently, microsatellite-instability (MSI) was discussed as genetic factor contributing to PCC development. Since microsatellite markers used for MSI detection have only been recommended for colorectal carcinoma (CRC), we established an extended marker set for MSI detection in PCC.Twenty-two PCC patients were analyzed applying 11 microsatellite markers. Our marker set comprised the reference panel for CRC and six additional markers, which have already been described to detect MSI in tumors other than CRC. Moreover, 23 endocrine tumors with gastrointestinal origin were examined in order to test the applicability of this marker panel.Microsatellite-instability was detected in 41% of PCCs. Twenty-seven percent showed loss of heterozygosity (LOH) events affecting different chromosomal regions. Among the 23 patients with endocrine tumors, only three (one pancreatic endocrine tumor, one duodenal neuro-endocrine tumor, one hepatic metastasis of a primary tumor with unknown origin) demonstrated MSI.The extended microsatellite panel is qualified to detect MSI in PCC. Nine percent of MSI-positive cases would have not been noticed by the use of the reference panel alone. PCCs are characterized by low frequency MSI pointing to failures in factors involved in DNA replication.