The alpha-subunit of the TRAP complex (TRAP alpha also known as signal sequence receptor 1/alpha subunit, SSRA)) is a single-spanning membrane protein of the endoplasmic reticulum (ER) which is found in proximity of nascent polypeptide chains translocating across the membrane [1,2]. TRAP complex consists of four subunits (alpha, beta, gamma, delta) and localises in the ER membrane. It associates with the Sec61 translocon as a heterotetramer [3]. It also interacts with palmitoylated calnexin (CALX), the interaction is required for efficient folding of glycosylated proteins [4]. TRAP complex may also be involved in endoplasmic reticulum-associated degradation [5]. TRAP-alpha plays a critical role in the biosynthesis of insulin, being involved in preproinsulin translocation and proinsulin trafficking which may contribute to the pathogenesis of type 2 diabetes [2]. This entry also includes IRC22 from fungi, which show sequence and structural similarities with TRAP-alpha. The function of IRC22 is not yet known.

1: The N-terminal region of the alpha-subunit of the TRAP complex has a conserved cluster of negative charges. Hartmann E, Prehn S; FEBS Lett 1994;349:324-326. PMID:8050590

2: Requirement for translocon-associated protein (TRAP) alpha in insulin biogenesis. Li X, Itani OA, Haataja L, Dumas KJ, Yang J, Cha J, Flibotte S, Shih HJ, Delaney CE, Xu J, Qi L, Arvan P, Liu M, Hu PJ; Sci Adv. 2019;5:eaax0292. PMID:31840061

3: Architecture of the ribosome-channel complex derived from native membranes. Menetret JF, Hegde RS, Heinrich SU, Chandramouli P, Ludtke SJ, Rapoport TA, Akey CW; J Mol Biol. 2005;348:445-457. PMID:15811380

4: Palmitoylated calnexin is a key component of the ribosome-translocon complex. Lakkaraju AK, Abrami L, Lemmin T, Blaskovic S, Kunz B, Kihara A, Dal Peraro M, van der Goot FG; EMBO J. 2012;31:1823-1835. PMID:22314232

5: Simultaneous induction of the four subunits of the TRAP complex by ER stress accelerates ER degradation. Nagasawa K, Higashi T, Hosokawa N, Kaufman RJ, Nagata K; EMBO Rep. 2007;8:483-489. PMID:17380188