Quaternary Structure of Polynucleotide Phosphorylase from Escherichia coli: Evidence of a Complex between Two Types of Polypeptide Chains

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Abstract
A new form of polynucleotide phosphorylase containing α and β 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, α, form B contains two types of chains: α (Mr 86000 ± 5000) and β (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 β chains; the role of the latter still remains to be specified.