Multiple cytosolic components promote intra-Golgi protein transport. Resolution of a protein acting at a late stage, prior to membrane fusion.

Cytosolic components are required to produce the "primed donor" and to consume the "dilution-resistant" intermediates of the intercompartmental protein transport pathway as elucidated in a cell-free system (Balch, W. E., Glick, B. S., and Rothman, J. E. (1984) Cell 39, 525-536, and Wattenberg, B. W., Balch, W. E., and Rothman, J. E. (1986) J. Biol. Chem. 261, 2202-2207). Widely different levels of crude cytosol are required for each of these steps, suggesting that different cytosolic components might mediate each step. Here, we fractionate cytosol and demonstrate that there are multiple transport-active components. Furthermore, we report the development of stage-specific functional assays which reveal that a distinct soluble component is required in the consumption of the dilution-resistant intermediate. This component, of about 25 kilodaltons in its apparent native molecular mass, is derived from calf brain cytosol. While this component mediates the consumption of the dilution-resistant intermediate, it is inactive in the priming stage. This stage-specific component seems likely to be involved in the processing of transport vesicles after the attachment of those vesicles to the target membranes.