Our goal was to better understand the interaction between the Hsp70 Ssb1 and the atypical Hsp70 Ssz1, which we had uncovered by in vivo site-specific Bpa crosslinking. Ssz1 having Bpa incorporated was purified and incubated with purified Ssb1 and the J-domain protein Zuo1 during activation of photo cross-linking. After protein gel electrophoresis and protease digestion of excised bands, the resulting cross-linked peptides were analyzed by mass spectrometry and identified by StavroX.