Identification and characterization of novel isoforms of COPI subunits

COP I-coated vesicles are involved in vesicular trafficking in the early secretory pathway. The COP I coat is composed of seven subunits, alpha-, beta-, beta'-, gamma-, delta-, epsilon-, and zeta -COPs. Evidence suggests, however, that there may be isoforms of the COP I subunits, In the present study, we identified homologs of gamma -COP (gamma2-COP; original gamma -COP is referred to as gamma1-COP in this paper) and of zeta -COP (zeta2-COP; original zeta -COP is referred to as zeta1-COP). gamma1-and gamma2-COPs, and zeta1- and zeta2-COPs share 80 and 75%, respectively, of amino acids. mRNAs for gamma2-COP and zeta2-COP are expressed ubiquitously, suggesting their fundamental role in cellular function. Immunofluorescence analysis shows that gamma2-COP and zeta2-COP are colocalized with beta -COP in the paranuclear cis-Golgi region. Yeast two-hybrid analysis indicates that gamma1- and gamma2-COPs can directly, albeit promiscuously, interact with zeta1- and zeta2-COPs. Like gamma1-COP, gamma2-COP can, form a complex with beta -COP in vivo, The gamma1-COP-containing and gamma2-COP-containing complexes can similarly interact with the cytoplasmic domain of p23, These results indicate that gamma2-COP and zeta2-COP can form a COP I-like complex in place of gamma1-COP and zeta1-COP, respectively, and suggest that the COP I complex and the COP I-like complex are functionally redundant.