XPA is susceptible to proteolytic cleavage by cathepsin L during lysis of quiescent cells

The xeroderma pigmentosum group A (XPA) protein plays an essential role in the removal of UV photoproducts and other bulky lesions from DNA as a component of the nucleotide excision repair (NER) machinery. Using cell lysates prepared from confluent cultures of human cells and from human skin epidermis, we observed an additional XPA antibody-reactive band on immunoblots that was approximately 3-4 kDa smaller than the native, full-length XPA protein. Biochemical studies revealed this smaller molecular weight XPA species to be due to proteolysis at the C-terminus of the protein, which negatively impacted the ability of XPA to interact with the NER protein TFIIH. Further work identified the endopeptidase cathepsin L, which is expressed at higher levels in quiescent cells, as the protease responsible for cleaving XPA during cell lysis. These results suggest that supplementation of lysis buffers with inhibitors of cathepsin L is important to prevent cleavage of XPA during lysis of confluent cells.