A novel hnRNP protein (HAP/SAF-B) enters a subset of hnRNP complexes and relocates in nuclear granules in response to heat shock

A two-hybrid screening in yeast for proteins interacting,vith the human hnR NP A1, yielded a nuclear protein of 917 amino acids that we termed hnRNP A1 associated protein (HAP), HAP contains an RNA binding domain (RBD) flanked by a negatively charged domain and by an S/K-R/E-rich region. In in vitro pull-down assays, HAP interacts with hnRNP A1, through its S/K-R/E-rich reg ion, and with several other hnRNPs. HAP was found to be identical to the pr eviously described Scaffold Attachment Factor B (SAF-B) and to HET, a trans criptional regulator of the Heat Shock Protein 27 gene. We show that HAP is a bona fide hnRNP protein, since anti-MAP antibodies immunoprecipitate fro m HeLa cell nucleoplasm the complete set of hnRNP proteins. Unlike most hnR NP proteins, the subnuclear distribution of HAP is profoundly modified in h eat-shocked HeLa cells, Heat-shock treatment at 42 degrees C causes a transcription-dependent recru itment of HAP to a few large nuclear granules that exactly coincide with si tes of accumulation of Heat Shock Factor 1 (HSF1), The recruitment of HAP t o the granules is temporally delayed with respect to HSF1 and persists for a longer time during recovery at 37 degrees C, The hnRNP complexes immunopr ecipitated from nucleoplasm of heat-shocked cells with anti-MAP antibodies have an altered protein composition with respect to canonical complexes. Al together our results suggest an involvement of HAP in the cellular response to heat shock, possibly at the RNA metabolism level.