HYDROPHOBIC REGION OF SIGNAL PEPTIDES IS A DETERMINANT FOR SRP RECOGNITION AND PROTEIN TRANSLOCATION ACROSS THE ER MEMBRANE

Newly synthesized mammalian secretory proteins such as preprolactin ap e translocated across the endoplasmic reticulum (ER) in a signal recog nition particle (SRP)-dependent manner. Recent studies revealed that t here are two recognition steps for signal peptides during this translo cation. The first step is recognition by SRP, which results in elongat ion arrest, and the second step is interaction between signal peptides and the translocation channel embedded in the En membrane. To determi ne the roles of the hydrophobic region of signal peptides in the recog nition by SRP and the membrane-embedded translocation machinery, we co nstructed chimeric proteins consisting of the mature region of preprol actin and signal peptides containing different numbers of leucine resi dues. The translocation of these chimeric proteins was completely depe ndent on SRP, and the efficiency increased as the number of leucine re sidues increased up to 10 and then decreased, Although the efficiency of elongation arrest also increased as the number of leucine residues increased up to 10, it only slightly decreased as the number increased up to 20. Similar results were obtained when the hydrophobic region w as replaced by alternate leucine and alanine residues, except that the most efficient translocation occurred when the number was 14. Taken t ogether, the present results suggest that the total hydrophobicity of the hydrophobic region of signal peptides is a determinant for recogni tion by both SRP and the membrane-embedded translocation machinery, al though the specificities of the two signal recognition steps are sligh tly different from each other.