Recombinant Mouse ASGR1/ASGPR1 Protein

The mouse asialoglycoprotein receptor (ASGP-R) is an endocytic recycling receptor that belongs to the long-form subfamily of the C-type/Ca⁺⁺-dependent lectin family (1 - 3). It is a complex of two noncovalently-linked subunits, a major 42 kDa glycoprotein (ASGPR1) and a minor 51 kDa glycoprotein (ASGR2). The major mouse ASGP-R subunit, ASGPR1, is synthesized as a 284 amino acid (aa) type II transmembrane (TM) protein that contains a 39 aa cytoplasmic region, a 21 aa TM segment, and a 224 aa extracellular domain (ECD) (4 - 6). The ECD contains two important structural regions. The first is a stalk region of 56 aa (aa’s # 59 - 117) that contributes to noncovalent oligomerization. The second is a 118 aa, carbohydrate-binding, Ca⁺⁺-dependent C-type lectin domain (aa’s 160 - 277) that is unusually stabilized by three Ca⁺⁺ ions (3, 5). There are two potential alternate splice forms for ASGPR1. Both are TM and show a deletion of the C-type lectin domain. One is 113 aa in length and shows a deletion of aa’s # 114 - 284 (7). The second is 132 aa in length and shows a deletion of aa’s 118 - 146 and aa’s 162 - 284 (8). Mouse ASGPR1 ECD is 89% and 79% aa identical to the ASGPR1 ECD in rat and human, respectively. The minor mouse ASGP-R subunit, ASGR2, is also a C-type lectin that shares the same structural organization as ASGR-1. It is 301 aa in length and has two 45 kDa and 51 kDa differentially-glycosylated isoforms (4, 6, 9). The ECD of ASGR2 is 50% aa identical to the ECD of ASGPR1. Although ASGPR1 and 2 can be expressed individually, a fully functional and stable ASGP-R requires simultaneous expression of both subunits (10 - 12). The stoichiometry of a functional ASGP-R is suggested to be either a 2:2, 3:1 or 3:2 ratio of ASGPR1:ASGR2 (13, 14). ASGPR1 is reported to bind Gal (nonreducing), GalNAc, and sialic acid alpha2,6GalNAc (3, 15, 16). This is generally in the context of triantennary or tetraantennary configurations (2).