Import of frog prepropeptide GLa into microsomes requires ATP but does not involve docking protein or ribosomes

Frog prepropeptide GLa, a precursor to a secretory protein containing 64 amino acids, was processed and imported by dog pancreas microsomes. These events did not depend on either docking protein or on the presence of ribosomes. A hybrid protein between the first 60 amino acids of prepropeptide GLa and an unrelated peptide of 49 amino acids fused to the carboxy terminus, however, behaved like a typical secretory protein precursor with regard to docking protein dependence. This suggests that independence of the need for docking protein, in the case of prepropeptide GLa, can be attributed to the size of the precursor protein. Processing and import of prepropeptide GLa by microsomes were ATP dependent. Therefore, import of proteins into the endoplasmic reticulum (ER) includes an ATP-requiring step not involving a ribosome/ribosome receptor or signal recognition particle (SRP)/docking protein interaction.