p23/Tmp21 Associates with Protein Kinase Cδ (PKCδ) and Modulates Its Apoptotic Function

Abstract

There is emerging evidence that C1 domains, motifs originally identified in PKC isozymes and responsible for binding of phorbol esters and diacylglycerol, interact with the Golgi/endoplasmic reticulum protein p23 (Tmp21). In this study, we investigated whether PKC␦, a kinase widely implicated in apoptosis and inhibition of cell cycle progression, associates with p23 and determined the potential functional implications of this interaction. Using a yeast two-hybrid approach, we found that the PKC␦ C1b domain associates with p23 and identified two key residues (Asp 245 and Met 266 ) implicated in this interaction. Interestingly, silencing p23 from LNCaP prostate cancer cells using RNAi markedly enhanced PKC␦-dependent apoptosis and activation of PKC␦ downstream effectors ROCK and JNK by phorbol 12-myristate 13-acetate. Moreover, translocation of PKC␦ to the plasma membrane by phorbol 12-myristate 13-acetate was enhanced in p23-depleted LNCaP cells. Notably, a PKC␦ mutant that failed to interact with p23 triggered a strong apoptotic response when expressed in LNCaP cells. In summary, our data compellingly support the concept that C1 domains have dual roles both in lipid and protein associations and provide strong evidence that p23 acts as an anchoring protein that retains PKC␦ at the perinuclear region, thus limiting the availability of this kinase for activation in response to stimuli. p23/Tmp21 (hereafter referred to as p23) is a type I transmembrane protein that belongs to the p24 endoplasmic reticulum/Golgi cargo family and plays critical roles in protein transport, organization of membrane structure, and the secretory pathway. Members of the p24 family have been widely implicated as COP (coat protein) vesicle cargo receptors, and they are involved in COP vesicle budding and the organization of the Golgi apparatus (1). In addition to the well established role of p23 in vesicle formation and cargo transport in the endoplasmic reticulum/Golgi (2-5), this protein has been implicated in several other cellular functions, including the recruitment of the small GTPase ADP-ribosylation factor 1 to the Golgi apparatus (6, 7) and ADP-ribosylation factor 1-dependent assembly of actin in the Golgi (8). p23 also is a component of the presenilin complex at the plasma membrane and modulates ␥-secretase cleavage (9).In a previous yeast two-hybrid screening, we established that p23 physically associates with ␣-and ␤-chimaerins, Rac GTPase-activating proteins (GAPs) 4 regulated by the lipid second messenger diacylglycerol (DAG) and DAG mimetic analogs such as the phorbol esters (10 -13). Mutational and deletional studies revealed that this interaction is mediated by the C1 domain present in chimaerins (10). The 50 -51-amino acid cysteine-rich C1 domains were identified originally in PKC isozymes as the sites for DAG and phorbol ester binding. C1 domains drive the recruitment of proteins to internal membranes and, in this manner, facilitate their access to docking proteins and effectors. More recently, we identified key residues w...

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