The Axin binding site on GSK-3 presumably overlaps with that for FRATtide; its proximity to the active site explains how Axin may act as a scaffold protein promoting beta-catenin phosphorylation. Tyrosine 216 phosphorylation can induce an active conformation in the activation loop. Pre-phosphorylated substrate peptides can be modeled into the active site of the enzyme, with the P1 residue occupying a pocket partially formed by phosphotyrosine 216 and the P4 phosphoserine occupying the 'primed' binding site.

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We have isolated and characterised diploid and tetraploid clones from the normally heterologous Spodoptera frugiperda (Sf-9) cell line by dilution cloning technique. Tetraploid clones were found to have cell sizes in excess of 35% larger than that of the diploid clones. In contrast, the maximum cell numbers achieved in batch cultures of diploid clones were on average 185% higher than the tetraploid cell numbers. Growth rates and metabolic quotients during the exponential phase were similar for both clones. Tetraploid cells infected with wild-type and recombinant green fluorescent protein (GFP) baculovirus, resulted in more polyhedra or GFP product per cell. Importantly, the difference between the clones either completely diminished or reduced to 50% when the yield was assessed in terms of the amount of polyhedra or GFP per mL of medium, respectively. These results indicate that the existing heterogeneity in insect cell populations with respect to ploidy level, are correlated to cell growth and product yield.

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