A Novel Ca2+-binding Protein, p22, Is Required for Constitutive Membrane Traffic

Abstract

We have identified a novel protein, p22, required for "constitutive" exocytic membrane traffic. p22 belongs to the EF-hand superfamily of Ca2+-binding proteins and shows extensive similarity to the regulatory subunit of protein phosphatase 2B, calcineurin B. p22 is a cytosolic N-myristoylated protein that undergoes conformational changes upon binding of Ca2+. Antibodies against a p22 peptide block the targeting/fusion of transcytotic vesicles with the apical plasma membrane, but recombinant wild-type p22 overcomes that inhibition. Nonmyristoylated p22, or p22 incapable of undergoing Ca2+-induced conformational changes, cannot reverse the antibody-mediated inhibition. The data suggest that p22 may act by transducing cellular Ca2+ signals to downstream effectors. p22 is ubiquitously expressed, and we propose that its function is required for membrane trafficking events common to many cells.

Expression of Calcineurin B Homologous Protein 2 Protects Serum Deprivation-induced Cell Death by Serum-independent Activation of Na+/H+ Exchanger

“…Ubiquitous CHP1 was previously suggested to be involved in various cell functions, such as inhibition of calcineurin activity (37), vesicular transport of proteins (38), interaction with microtubules (39), and interaction with a death-associated protein kinase-related apoptosis-inducing protein kinase 2 (DRAK2) (40). Although we do not know whether CHP2 is also involved in these cellular functions, they may not be relevant to the observed effects of CHP2 because the functions of CHP1/NHE1 and CHP2/NHE1 were compared in this study.…”

Regulation of Intra-Golgi Membrane Transport by Calcium

“…23 and 24). More recently it was reported that Ca 2ϩ participates in other intracellular transport events, including ER to Golgi transport (25), assembly of nuclear membrane (10), transcytotic vesicle fusion (26), vacuolar membrane fusion (11), and fusion between endosomes (12, 13). The requirement for Ca 2ϩ in intra-Golgi transport has remained unclear, however, because early reports using a cellfree intra-Golgi transport assay demonstrated no effect of EGTA (14), whereas in semi-intact cells, EGTA was able to inhibit this transport process (9).…”

Acute Regulation of Na/H Exchanger NHE3 by Adenosine A1 Receptors Is Mediated by Calcineurin Homologous Protein

“…The ability of CHP to alter NHE1 transport independent of surface NHE1 protein (28) renders it an attractive candidate for supporting adenosine-mediated change in intrinsic transport activity of NHE3. CHP associates with microtubules via an N-myristoylation-dependent mechanism that does not involve conventional microtubule-associated proteins (30) and is involved in vesicular transport (25). CHP also interacts with kinesin-related protein KIF1B␤2, a molecular motor in neurons (31).…”

“…Calcineurin homologous protein (CHP) is a recently described widely expressed Ca 2ϩ -binding EF-hand protein of ϳ22 kDa (25,26) with significant similarity to the regulatory B subunit of the heterodimeric protein phosphatase, calcineurin (26). CHP inhibits calcineurin phosphate activity (27) and interacts with NHEs (26,28,29).…”