ABSTRACT
Secretory proteins are synthesized on membrane-bound ribosomes and translocated across the membrane of the rough endoplasmic reticulum (RER). While in the RER and the Golgi complex, secretory proteins are modified by a number of co- as well as posttranslational processes. The signal sequences are in most cases found at the amino termini of the nascent polypeptides, and they are rapidly removed from the polypeptides during the cotranslational transfer process. Many of the experiments suggested that collagen polypeptides were synthesized on membrane-bound polysomes, but the technical difficulties in preparing homogeneous subcellular fractions caused many investigators to interpret the data with caution. Hydroxylations of prolyl and lysyl residues by three hydroxylases located in the RER are the best characterized modifications of procollagen. Autoradiographic studies have been employed to localize intracellular sites of collagen synthesis while simultaneously tracing its secretion pathway. The synthesis, intracellular transport, and packaging of procollagen represents a unique variation on the general theme of protein secretion.
