The Incorporation of the First Glucose into the Tyrosine Residue of Glycogenin

S. BAZÁN; A.L. MONQAUT; J.M. ROMERO; J.L. BARRA; C.M. FIGUEROA; A.A. IGLESIAS; J.A. CURTINO

Iguazú, Misiones, Argentina

Congreso; XL Reunión de la Sociedad Argentina de Investigación en Bioquímica y Biología Molecular (SAIB); 2004

SAIB

ES-P16.THE INCORPORATION OF THE FIRST GLUCOSE INTO THE TYROSINE RESIDUE OF GLYCOGENINBazán S, Monqaut AL, Romero JM, Barra JL, Figueroa CM*, Iglesias AA*, Curtino JA.Dep. Quím. Biol.-CIQUIBIC, Fac. Cs. Quím.-CONICET,UNC, 5000 Córdoba, and *Lab. Enzim. Molec, Fac. Bioq Cs. Biol., UNL, S3000ZAA, Santa Fé, Argentina. E-mail: sole@dqb.fcq.unc.edu.arTo establish the glucosylation state of glycogenin (Gn) is relevant to perform studies for the understanding of its action mechanism.According to Alonso et al. (FEBS Lett., 352, 222, 1994),recombinant Gn is expressed in E. coli galU, lacking UDPG pyrophosphorylase, as carbohydrate-free apoGn. Evidence for the apo form was a faster SDS-PAGE mobility than glucosylated Gn expressed in the wt, and the failure to detect linked carbohydrate.Previously (SAIB 2003), we described a way to compare the glucosylation state under which recombinant Gn is expressed, by using the auto-[14C]glucosylation/trans-[14C]glucosylation index (ATI) of both, the intact and exhaustive amylolyzed forms. We now describe that Gn expressed in the galU mutant (U-Gn) shows the fast SDS-PAGE mobility ascribed to apoGn, but the ATI values indicate that U-Gn is expressed in a glucosylated form. Moreover, the galU strain shows both, UDPG- and CDPG-pyrophosphorylase activities (Upy and Cpy). The Upy was 1/3-1/5 the activity of the wt, which together with Cpy could sustain Gn glucosylation. Nocontrols of the lack of Upy and Cpy were shown by Alonso et al., as to exclude any  glucosylated Gn as the autoglucosylating species of U-Gn rather than apoGn incorporating also the first glucose to its tyrosine residue. We conclude that the Gn autoglucosylation of Tyr-194 deserves further evidence, the nature of the reaction still remaining an open question.