This calcium sensor plays a critical role in various cellular processes, including endoplasmic reticulum (ER)-Golgi vesicular transport, endosomal biogenesis, and membrane repair. It functions as an adaptor protein, bridging disparate proteins or stabilizing weak protein-protein interactions in response to calcium fluctuations. Calcium binding exposes a hydrophobic surface, facilitating interactions with diverse protein sets via three distinct hydrophobic pockets, ultimately leading to membrane translocation. Specifically, it's involved in ER-Golgi transport by promoting the interaction between PDCD6IP and TSG101, linking ESCRT-III and ESCRT-I complexes. In conjunction with PEF1, it acts as a calcium-dependent adaptor for the BCR (KLHL12) complex, regulating COPII coat size and thus influencing ER-Golgi transport. Upon cytosolic calcium increase, its heterodimer with PEF1 interacts with and bridges the BCR (KLHL12) complex and SEC31 (SEC31A or SEC31B), promoting SEC31 monoubiquitination and subsequent collagen export, essential for neural crest specification. It also regulates MCOLN1 distribution and function within the endosomal pathway, promotes TFG localization and polymerization at ER exit sites, and is crucial for T-cell receptor-, Fas-, and glucocorticoid-induced apoptosis. It may mediate calcium-regulated signals in the apoptotic pathway, potentially accelerating cell death by increasing caspase-3 activity via interaction with DAPK1, although its role in apoptosis may be indirect. Additionally, it may inhibit KDR/VEGFR2-dependent angiogenesis by inhibiting VEGF-induced Akt pathway phosphorylation. In HIV-1 infection, it indirectly inhibits viral production by modulating viral Gag expression and distribution. This isoform exhibits a lower calcium affinity compared to isoform 1.
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