In gastric oxyntic cells, the trafficking of the H,K-ATPase between a subapical tubulovesicular compartment and the apical membrane occurs in a secretagogue-specific manner. The underlying mechanism of this process is largely uncharacterized. Insight in this process is likely to be relevant to the apical recycling pathway of other epithelial cells. We have immunologically identified the heavy chain of clathrin on oxyntic cell tubulovesicles. Clathrin is a vesicular coat protein that has been demonstrated to play a role in the internalization of membrane proteins from the plasma membrane and in the formation of transport vesicles from the trans-Golgi network. The identification of clathrin on tubulovesicles is intriguing, given the observation that typical assembled clathrin coats on tubulovesicles have not been observed by electron microscopy. Thus, oxyntic cell tubulovesicular clathrin may represent a novel isoform of clathrin. This isoform may assemble in a different manner resulting in a morphologically indistinct vesicular coat. Alternatively, other coat proteins modulating clathrin assembly, such as clathrin light chains and clathrin adaptor complexes, may be unique isoforms. Thus, to aid in the molecular characterization of the components of the clathrin-containing tubulovesicular coat, we propose to obtain peptide sequences from tryptic peptides by mass spectrometry. The proteins of interest include: clathrin heavy chain, clathrin light chain, and clathrin adaptors. The peptide sequences will provide immediate characterization of the proteins of interest. In addition, they may be used to generate anti-peptide antibodies and oligonucleotide probes for molecular cloning of the proteins of interest.