Quaternary structure of polynucleotide phosphorylase from Escherichia coli: evidence of a complex between two types of polypeptide chains.

A new form of polynucleotide phosphorylase containing alpha and beta subunits was isolated (form B) by purification without preparative electrophoresis; this form was compared to the enzyme obtained by preparative electrophoresis purification (form A). The Stokes radius of these two forms are very different: 6.4 nm for form A and 8.7 - 9.0 nm for form B; on the other hand the sedimentation coefficients are close: 8.9 S and 9.9 S respectively. Such a result suggests that form B is very asymmetric. The apparent molecular weights, calculated from the Stokes radii and from the sedimentation coefficients, are approximately 365000 for form B and 252000 for form A. The latter is homogeneous on polyacrylamide gel, whereas the former yields two components, one of which behaves similarly to form A. Finally, whereas form A is composed of only one type of subunit, alpha, form B contains two types of chains: alpha (Mr 86000 +/- 5000) and beta (Mr 48000 +/- 2000) in stoichiometric proportions. From these results we believe that one should assume for form B the existence of a complex between form A and beta chains; the role of the latter still remains to be specified.