The ATP requiring step in assembly of M13 procoat protein into microsomes is related to preservation of transport competence of the precursor protein.

M13 procoat protein is processed to transmembrane coat protein by dog pancreas microsomes after completion of synthesis and in the absence of the signal recognition particle (SRP)/docking protein system. ATP is required for fast and efficient processing of procoat protein by microsomes in a reticulocyte lysate. Requirement for ATP is also observed in the absence of ribosomes or docking protein. This indicates the existence of a unique assembly pathway for procoat protein into microsomes which depends on ATP but does not depend on the SRP/docking protein and ribosome/ribosome receptor systems. We suggest that the ATP requirement is linked to a so far unknown component of the reticulocyte lysate, acting on transport competence of precursor proteins.