Enhanced association of mutant triosephosphate isomerase to red cell membranes and to brain microtubules

In a Hungarian family with triosephosphate isomerase (TPI; D-glyceraldehyde -3-phosphate keto-isomerase, EC 5.3.1.1) deficiency, two germ-line identica l, but phenotypically differing compound heterozygote brothers tone of them with neurological disorder) have been identified with the same very low (< 5%) TPI activity and 20- or 40-fold higher erythrocyte dihydroxyacetone ph osphate levels as compared with normal controls. Our present studies with p urified TPI and hemolysates revealed the binding of TPI, and the binding of human wild-type and mutant TPIs in hemolysate, to the red cell membrane, a nd the interference of binding with other hemolysate proteins. The binding of the mutant TPI is enhanced as compared with the wild-type enzyme. The in creased binding is influenced by both the altered structure of the mutant a nd the changes in the red cell membrane. Compared with binding of glycerald ehyde-3-phosphate dehydrogenase, the isomerase binding is much less sensiti ve to ionic strength or blocking of the N-terminal tail of the band-3 trans membrane protein. The binding of TPIs to the membrane decreases the isomera se activity, resulting in extremely high dihydroxyacetone phosphate levels in deficient cells. In cell-free brain extract, tubulin copolymerizes with TPI and with other cytosolic proteins forming highly decorated microtubules as shown by immunoblot analysis with anti-TPI antibody and by electron mic roscopic images. The efficacy order of TPI binding to microtubules is propo situs > brother without neurological disorder > normal control. This distin ct microcompartmentation of mutant proteins may be relevant in the developm ent of the neurodegenerative process in TPI deficiency and in other, more c ommon neurological diseases.