Identification and Characterization of Novel Isoforms of COP I Subunits.

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COP I-coated vesicles are involved in vesicular trafficking in the early secretory path-way. The COP I coat is composed of seven subunits, α- β-, β-, γ-, δ-, ε-, and ζ-COPs. Evi-dence suggests, however, that there may be isoforms of the COP I subunits. In the present study, we identified homologs of γ-COP (γ2-COP; original γ-COP is referred to as γ1-COP in this paper) and of ζ-COP (ζ2-COP; original ζ-COP is referred to as ζ1-COP). γ1-and γ2-COPs, and ζ1- and ζ2-COPs share 80 and 75%, respectively, of amino acids. mRNAs for γ2-COP and ζ2-COP are expressed ubiquitously, suggesting their fundamental role in cellular function. Immunofluorescence analysis shows that γ2-COP and ζ2-COP are colocalized with β-COP in the paranuclear cis-Golgi region. Yeast two-hybrid analysis indicates that γ1- and γ2-COPs can directly, albeit promiscuously, interact with ζ1- and ζ2-COPs. Like γ1-COP, γ2-COP can form a complex with β-COP in vivo. The γ1-COP-containing and γ2-COP-containing complexes can similarly interact with the cyto-plasmic domain of p23. These results indicate that γ2-COP and ζ2-COP can form a COP I-like complex in place of γ1-COP and U-COP, respectively, and suggest that the COP I complex and the COP I-like complex are functionally redundant.
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