Catabolism of the Seed Storage Proteins From Lupinus Albus: Fate of Globulins During Germination and Seedling Growth.

Abstract

The degradation of the major seed storage globulins of Lupinus albus L. was studied during the first 11 days after the onset of germination. The collagenolytic activity present in cotyledonary extracts was found to closely follow the rate and pattern of total globulin degradation. Little or no degradation of the three main globulins was observed during the first days of germination and seedling growth. The rate of proteolysis of the ³-conglutin main subunit, composed of one heavy glycopolypeptide chain and one light non-glycosylated polypeptide chain, occurs gradually between days 3 and 11, with no apparent change in the globulin structure being detected during this time. Between days 3 and 5, ²-conglutin, with at least 14 of its heavier polypeptide chains glycosylated, suffers a dramatic change in its structure and concentration, involving the appearance of a new set of polypeptides including a higher molecular mass group, whose concentration steadily declines until complete disappearance after 11 days, and a lighter molecular mass group, whose concentration surprisingly increases from 5 to 11 days. Finally, ±-conglutin, with each of its four main subunits composed of a heavy glycopolypeptide chain and a 19 kDa non-glycosylated polypeptide chain, undergoes a pattern of degradation intermediate to those described for ³- and ²-conglutins in the sense that the new set of polypeptides coexist with polypeptides that were already present in the dry seed. The degradation of a-conglutin subunits starts at the level of their heavier polypeptide chains, releasing the intact 19 kDa polypeptide which gradually disappears after day 7. Our results indicate that ³-, ²- and ±-conglutins are subjected to differential proteolysis during germination and seedling growth. Furthermore, the different polypeptide chains of each conglutin are also degraded at significantly different rates.