SMIM1, carrier of the Vel blood group, is a tail-anchored transmembrane protein and readily forms homodimers in a cell-free system

Abstract

Antibodies to the Vel blood group antigen can cause adverse hemolytic reactions unless Vel-negative blood units are transfused. Since the genetic background of Vel-negativity was discovered in 2013, DNA-based typing of the 17-bp deletion causing the phenotype has facilitated identification of Vel-negative blood donors. SMIM1, the gene underlying Vel, encodes a 78-amino acid erythroid transmembrane protein of unknown function. The transmembrane orientation of SMIM1 has been debated since experimental data supported both the N- and C-termini being extracellular. Likewise, computational predictions of its orientation were divided and potential alternatives such as monotopic or dual-topology have been discussed but not investigated. We used a cell-free system to explore the topology of SMIM1 when synthesized in the endoplasmic reticulum (ER). SMIM1 was tagged with an opsin-derived N-glycosylation reporter at either the N- or C-terminus and synthesized in vitro using rabbit reticulocyte lysate supplemented with canine pancreatic microsomes as a source of ER membrane. SMIM1 topology was then determined by assessing the N-glycosylation of its N- or C-terminal tags. Complementary experiments were carried out by expressing the same SMIM1 variants in HEK293T/17 cells and establishing their membrane orientation by immunoblotting and flow cytometry. Our data consistently indicate that SMIM1 has its short C-terminus located extracellularly and that it most likely belongs to the tail-anchored class of membrane proteins with the bulk of the polypeptide located in the cytoplasm. Having established its membrane orientation in an independent model system, future work can now focus on functional aspects of SMIM1 as a potential regulator of erythropoiesis.

Identification of SMIM1 and SEZ6L2 as Potential Biomarkers for Genes Associated with Intervertebral Disc Degeneration in Pyroptosis

“…The small integral membrane protein 1 (SMIM1) is a limited size red blood cell (RBC) membrane protein whose structure is not thoroughly understood and which is associated with a variety of immune responses [ 44 , 45 ]. It plays a prominent role in RBC differentiation [ 46 ].…”

Downregulation of SMIM3 inhibits growth of leukemia via PI3K-AKT signaling pathway and correlates with prognosis of adult acute myeloid leukemia with normal karyotype

“…SMIM families contain multiple members. Small integral membrane protein 1 (SMIM1), a tail-anchored transmembrane protein [17], is associated with Vel-negative blood type [18,19]. SMIM1 also could influence red blood cell traits [20].…”