Melanocortin 3 Receptor Has a 5′ Exon That Directs Translation of Apically Localized Protein From the Second In-Frame ATG

Abstract

Melanocortin-3 receptor (MC3R) is a canonical MSH receptor that plays an essential role in energy homeostasis. Variants in MC3R have been implicated in obesity in humans and mice. However, interpretation of the functional consequences of these variants is challenging because the translational start site of MC3R is unclear. Using 5' rapid amplification of cDNA ends, we discovered a novel upstream exon that extends the length of the 5' untranslated region (UTR) in MC3R without changing the open-reading frame. The full-length 5' UTR directs utilization of an evolutionarily conserved second in-frame ATG as the primary translation start site. MC3R synthesized from the second ATG is localized to apical membranes of polarized Madin-Darby canine kidney cells, consistent with its function as a cell surface mediator of melanocortin signaling. Expression of MC3R causes relocalization of melanocortin receptor accessory protein 2, an accessory factor for melanocortin-2 receptor, to the apical membrane, coincident with the location of MC3R. In contrast, protein synthesized from MC3R cDNAs lacking the 5' UTR displayed diffuse cytosolic distribution and has no effect on the distribution of melanocortin receptor accessory protein 2. Our findings demonstrate that a previously unannotated 5' exon directs translation of MC3R protein that localizes to apical membranes of polarized cells. Together, our work provides insight on the structure of human MC3R and reveals a new pathway for regulation of energy metabolism.

Polymorphisms and mutations in the melanocortin-3 receptor and their relation to human obesity

“…Studies have evaluated the transcript structure of murine Mc3r and human MC3R by examining its untranslated regions (UTRs) [13, 14], which can play important roles in gene expression, mRNA stability, and translational efficiency. Research from our group [14] demonstrated that the murine Mc3r 5′ UTR has two transcription start sites: one 368 bases upstream of the translation start site and another 440 bases upstream, with putative initiator sequences associated with each start site.…”

“…The human MC3R 5′ UTR, on the other hand, has a transcription start site 527–544 bases upstream of the classical translation start site, with most transcripts starting 533 bases upstream, near a putative initiator sequence. There is also a 248 base splice from 140 to 388 bases upstream of the start codon between consensus splice donor and acceptor sites [13, 14]. The human MC3R 3′ UTR was found to end only 115–160 bases after the translational stop codon [13, 14].…”

“…There is also a 248 base splice from 140 to 388 bases upstream of the start codon between consensus splice donor and acceptor sites [13, 14]. The human MC3R 3′ UTR was found to end only 115–160 bases after the translational stop codon [13, 14]. The human MC3R has more than one potential start site for translation, with additional in-frame ATGs located 37 and 109 codons downstream.…”

“…The human MC3R has more than one potential start site for translation, with additional in-frame ATGs located 37 and 109 codons downstream. The first ATG is found only in humans and nonhuman primates, while the second ATG appears widely conserved among vertebrates [13]. Park et al reported that the full-length 5’ UTR directs utilization of the second in-frame ATG as the primary translation start site instead of the canonically-assumed first ATG [13].…”

“…The first ATG is found only in humans and nonhuman primates, while the second ATG appears widely conserved among vertebrates [13]. Park et al reported that the full-length 5’ UTR directs utilization of the second in-frame ATG as the primary translation start site instead of the canonically-assumed first ATG [13]. This leads to a protein transcript which is 37 codons shorter than the classical MC3R transcript [15]; although other research suggests that translation can occur from the first ATG site as well [16].…”