Purification and internal amino acid sequence of the 80 kDa protein kinase C substrate from Swiss 3T3 fibroblasts

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Abstract
The acidic 80 kDa protein kinase C (PKC) substrate was purified from 2.3 × 1010 Swiss 3T3 fibroblasts. Partial amino add sequence data were obtained from five peptides generated by S. aureus V8 cleavage of the protein, enabling a total of 91 amino acid residues to be assigned. The sequences of these five peptides were compared to the deduced amino acid sequences of acidic 80–87 kDa PKC substrates from both actively proliferating A431 epidermal carcinoma cells, and fully differentiated neural tissue. Despite their similar physical properties, there was no homology between the peptides derived from the fibroblast 80 kDa protein and the PKC substrate from A431 cells. However, there was 66% homology with the 87 kDa bovine brain protein within the regions covered by the peptides (about 30% of the total protein). Furthermore, comparison of the peptides from the fibroblast 80 kDa protein with proteolytic peptides derived from the acidic 80 kDa rat brain protein revealed an overall homology of 89%. These data provide the first direct evidence that the 80 kDa PKC substrate from Swiss 3T3 fibroblasts is closely related to the 80–87 kDa PKC substrates detected in fully differentiated neural tissue.

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