Evidence that fructose 1,6-bisphosphate specifically protects the alpha-subunit of pyrophosphate-dependent 6-phosphofructo-1-phosphotransferase against proteolytic degradation.

Pyrophosphate-dependent 6-phosphofructo-1-phosphotransferase (PFP) consists of alpha (regulatory) and beta (catalytic) subunits. The alpha-subunit was previously reported to be much more susceptible to tryptic digestion than the beta-subunit. In this study, ligand-induced protection of PFP subunits against proteolysis by subtilisin was investigated in vitro and the data obtained demonstrated that fructose 1,6-bisphosphate (Fru-1,6-P(2)), while exerting negligible effect on the beta-subunit, remarkably protected the alpha-subunit against proteolytic degradation. Western blot analysis revealed a good correlation between the Fru-1,6-P(2) concentration and the degree of corresponding protection on the alpha-subunit against proteolysis. In contrast, none of other examined ligands including fructose 2,6-bisphosphate, fructose 6-phosphate and pyrophosphate had such protection on the alpha-subunit. This finding (1) indicates that the stability of the alpha-subunit can be selectively increased by Fru-1,6-P(2), and (2) suggests that Fru-1,6-P(2) is likely a special effector of the alpha-subunit.