In early secretory transport, coat recruitment for the formation of coat protein I (COPI) vesicles involves binding to donor Golgi membranes of the small GTPase ADP-ribosylation factor 1 and subsequent attachment of the cytoplasmic heptameric complex coatomer. Various hypotheses exist as to the precise role of and possible routes taken by COPI vesicles in the mammalian cell. Here we report the ubiquitous expression of two novel isotypes of coatomer subunits ␥-and -COP that are incorporated into coatomer, and show that three isotypes exist of the complex defined by the subunit combinations ␥1/1, ␥1/2, and ␥2/1. In a liver cytosol, these forms make up the total coatomer in a ratio of about 2:1:2, respectively. The coatomer isotypes are located differentially within the early secretory pathway, and the ␥2/1 isotype is preferentially incorporated into COPI vesicles. A population of COPI vesicles was characterized that almost exclusively contains ␥2/1 coatomer. This existence of three structurally different forms of coatomer will need to be considered in future models of COPI-mediated transport.

SummaryIt is well established that assembly of the peripheral antenna complex, LH2, is required for proper photosynthetic membrane biogenesis in the purple bacterium Rhodobacter sphaeroides.

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