Characterization of c-myc proteins from avian bursal lymphoma cell lines

We have used a rabbit antiserum directed against a portion of the MC29 viral myc protein expressed in bacteria to characterize the cellular myc protein from three different avian bursal lymphoma cell lines (1104HI, 1104BI S13, BK25), and from normal chick embryo cells. The phosphorylated myc proteins immunoprecipitated from these cells varied in molecular weight from 58 to 62 kDa and localized to the cell nucleus, as shown by cell fractionation experiments. Pulse-chase experiments established that these proteins had short half-lives ranging from 12 min for the myc proteins from the 1104BI S13 cell line to 25 min for myc proteins from both the 1104HI and the BK25 cell lines. The structural relatedness of the proteins was established by comparing their partial proteolytic digestion products (Cleveland analysis) with the partial proteolytic digestion products of the MH2 viral myc protein. The anti-myc-serum also immunoprecipitated a 48-kDa protein from each of the bursal cell lines. We have identified this protein as a breakdown product of the bursal cell myc proteins. The different size and number of these bursal cell myc proteins may be a direct result of the specific site of integration as well as the orientation of the retrovirus LTR sequence relative to the adjacent cellular myc allele.