A24
Multiple isoforms of HAX1 in murine and human tissues suggest a novel adaptor family of proteins
Delphine M Lees, Ian R Hart, John F Marshall
Centre for Tumour Biology, Cancer Research-UK Clinical Centre, Bart's and The London, London, UK
Background
HAX1 (HS1-associated protein X-1) has been identified as a binding partner for many different cellular proteins. Functionally it participates in control of apoptosis, intracellular localisation of various proteins and mRNAs, and membrane abundance and endocytosis of transmembrane proteins, even though it lacks any enzymatic activity, or recognisable protein:protein binding motif. HAX1 also regulates Gβ13-dependent cell migration, and αvβ6-dependent migration and invasion. Moreover, different reports also have shown discrepancy in the intracellular localisation of HAX1. Thus the likelihood that a single molecule could perform these multiple functions and exist in multiple compartments seemed unlikely.
Method and results
We conducted a bioinformatics search of human and mouse databases for HAX1 predicted products. We identified multiple possible HAX1 isoforms in human and mouse tissues, predicted to be generated as splice variants. We designed isoform-specific primers for these putative products and isolated RNA from >30 primary and established cell lines. Using PCR we identified eight and two HAX1 isoforms in human and murine cell extracts, respectively. The products were sequenced and the corresponding proteins predicted. Interestingly, the mouse genome has two functional HAX1 genes, each of which potentially can generate an identical protein. We also detected several HAX1 proteins by Western blotting of human cell lysates and confirmed their identity as HAX1 variants by noting their reduction after RNA interference.
Conclusion
Our data suggest that HAX1 comprises a novel family of adaptor proteins and offers a molecular explanation for the wide variety of HAX1 functions, binding partners and intracellular localisations observed so far.
