Threading analysis suggests that the obese gene product may be a helical cytokine

The ob gene encodes a protein that, in mutant form, is associated with obesity and type II diabetes in mice. Sequence analysis has revealed no similarities to other proteins, however, and no clues as to possible functions. The possibility nonetheless remains that ob is functionally or ancestrally related to other proteins, whose sequences are divergent to the point that only a comparison of three-dimensional structures might detect relationship. To explore this possibility, we conduct a 'threading' search of a 3-dimensional structure database, to determine whether the ob protein might adopt a fold similar to any known structure. This search reveals that the ob sequence is compatible, at a significance level of P < 0.05, with structures from the family of helical cytokines that includes interleukin-2 and growth hormone. A structural model of ob based upon these results is physically and biologically plausible and leads to testable predictions, including the prediction that ob may activate the JAK-STAT pathway, via binding to a receptor resembling those of the cytokine family.