Molecular size, subunit structure and microheterogeneity of glycoprotein II from the seeds of kidney bean (Phaseolus vulgaris L.).

The isolation of Glycoprotein II from the seeds of kidney bean (Phaseolus vulgaris) cv. 'Processor' is described. This glycoprotein was shown by SDS-gel electrophoresis to dissociate into four subunits, 53 000, 50 000, 47 000 and 43 000 (alpha : beta : gamma : delta), in an approximate ratio of 2 : 0.2 : 2 : 1. At neutral and slightly alkaline pH values its molecular weight was about 142 000 (protomer) while at pH 5 it was mainly in the form of a tetramer with a molecular weight value of about 560 000. Samples of Glycoprotein II were shown by isoelectric focusing, molecular sieve chromatography and immunochemical methods to be microheterogenous. A number of fractions were prepared by these methods in which the proportion of the major subunits (alpha : gamma : delta) varied between 1 : 1 : 1 to 3 : 3 : 1. These ratios were also shown to change during development of the seed. On the basis of these results it is suggested that, at and above pH 7, samples of Glycoprotein II consist of microheterogenous population of molecules each containing three subunits per protomer. However, in these protomers the subunits are drawn, in different ratios, from the four subunits available. When the net molecular charge is small, at and around the isoelectric zone of Glycoptotein II, the protomer is converted mainly into the more stable tetramer containing 12 subunits.