THE TOMATO HSF SYSTEM - HSFA2 NEEDS INTERACTION WITH HSFA1 FOR EFFICIENT NUCLEAR IMPORT AND MAY BE LOCALIZED IN CYTOPLASMIC HEAT-STRESS GRANULES

In heat-stressed (HS) tomato (Lycopersicon peruvianum) cell cultures, the constitutively expressed HS transcription factor HsfA1 is compleme nted by two HS-inducible forms, HsfA2 and HsfB1. Because of its stabil ity HsfA2 accumulates to fairly high levels in the course of a prolong ed HS and recovery regimen, Using immunofluorescence and cell fraction ation experiments, we identified three states of HsfA2: (i) a soluble, cytoplasmic form in preinduced cultures maintained at 25 degrees C, ( ii) a salt-resistant, nuclear form Found in HS cells, and (iii) a stor ed form of HsfA2 in cytoplasmic HS granules, The efficient nuclear tra nsport of HsfA2 evidently requires interaction wie;a HsfA1, When expre ssed in tobacco protoplasts by use of a transient-expression system, H sfA2 is mainly retained in the cytoplasm unless it is coexpressed with HsfA1. The essential parts for the interaction and nuclear cotranspor t of the two Hsfs are the homologous oligomerization domain (HR-A/B re gion of the A-type Hsfs) and functional nuclear localization signal mo tifs of both partners, Direct physical interaction of the two Hsfs a w ith formation of relatively stabile Hetero-oligomers was shown by a tw o-hybrid test in Saccharomyces cerevisiae as well as by coimmunoprecip itation using tomato and tobacco whole-cell lysates.