The tsr1-1 mutation has been initially identified as an extragenic suppressor of the scr2.II-13 mutation that alters the 7SL RNA component of the signal recognition particle (SRP) and results in severe defects in protein translocation and SRP stability. We showed previously that the TSR1 gene was essential and that the tsr1-1 mutation allowed complete recovery of scr2.II-13-associated secretory defects. We show here that the tsr1-1 mutation also restores SRP stability in an scr2.II-13 context. The TSR1 gene product (Tsr1p) is stably associated with rapidly sedimenting material and cofractionates with the lumenal protein Kar2p of the endoplasmic reticulum; it behaves in protease protection assays as a transmembrane component. Coimmunoprecipitation experiments revealed a physical interaction with Kar2p and with ribosomal components associated to the 5.8S rRNA as well as with SRP components like Sec65p and 7SL RNA. We propose that Tsr1p is an important component of the endoplasmic reticulum membrane, interacting both with the SRP-ribosome complex in the cytosol and with Kar2p in the lumen of the endoplasmic reticulum.