Functional Characterization of the X-Linked Inhibitor of Apoptosis (XIAP) Internal Ribosome Entry Site Element: Role of La Autoantigen in XIAP Translation

Abstract

X-linked inhibitor of apoptosis protein (XIAP) is a key regulator of programmed cell death triggered by various apoptotic triggers. Translation of XIAP is controlled by a 162-nucleotide (nt) internal ribosome entry site (IRES) element located in the 5′ untranslated region of XIAP mRNA. XIAP IRES mediates efficient translation of XIAP under physiological stress and enhances cell protection against serum deprivation and radiation-induced apoptosis. In the present report we describe the assembly of a sequence-specific RNA-protein complex consisting of at least four cytosolic proteins on the XIAP IRES element. We determine that the core binding sequence is approximately 28 nt long and is located 34 nt upstream of the initiation site. Moreover, we identify the La autoantigen as a protein that specifically bindsXIAP IRES in vivo and in vitro. The biological relevance of this interaction is further demonstrated by the inhibition of XIAP IRES-mediated translation in the absence of functional La protein. The results suggest an important role for the La protein in the regulation of XIAP expression, possibly by facilitating ribosome recruitment to the XIAP IRES.

ACKNOWLEDGMENTS

We thank the members of our laboratory for useful discussion. We are grateful to N. Sonenberg for the generous gift of GST-La and GST-La(226–348) plasmid constructs and to E. Chan for the gift of anti-La A1 antibody.

This work was supported by grants from the Medical Research Council of Canada (MRC), the Canadian Networks of Centers of Excellence (NCE), and the Howard Hughes Medical Institute (HHMI). M.H. is a recipient of an MRC Postdoctoral Fellowship. R.G.K. is a recipient of an MRC Senior Scientist Award, a Fellow of the Royal Society of Canada, and an HHMI International Research Scholar.