Alternative splicing in the ligand binding domain of mouse ApoE receptor-2produces receptor variants binding reelin but not alpha(2)-macroglobulin

LR7/8B and ApoER2 are recently discovered members of the low density lipopr otein (LDL) receptor family, Although structurally different, these two pro teins are derived from homologous genes in chicken and man by alternative s plicing and contain 7 or 8 LDL receptor ligand binding repeats. Here we pre sent the cDNA for ApoER2 cloned from mouse brain and describe splice varian ts in the ligand binding domain of this protein, which are distinct from th ose present in man and chicken, The cloned cDNA is coding for a receptor wi th only five LDL receptor ligand-binding repeats, i.e. comprising repeats 1 -3, 7, and 8. Reverse transcriptase-polymerase chain reaction analysis of m RNA from murine brain revealed the existence of two additional transcripts. One is lacking repeat 8, and in the other repeat 8 is substituted for by a 13-amino acid insertion with a consensus site for furin cleavage arising f rom an additional small exon present in the murine gene. None of the transc ripts in the mouse, however, contain repeats 4-6. In murine placenta only t he form containing repeats 1-3 and 7 and the furin cleavage site is detecta ble. Analysis of the corresponding region of the murine gene showed the exi stence of 6 exons coding for a total of 8 ligand binding repeats, with one exon encoding repeats 4-6. Exon trapping experiments demonstrated that this exon is constitutively spliced out in all murine transcripts, Thus, the mu rine ApoER2 gene codes for receptor variants harboring either 4 or 5 bindin g repeats only. Recombinant expression of the 5-repeat and 4-repeat variant s showed that repeats 1-3, 7, and 8 are sufficient for binding of beta -ver y low density lipoprotein and reelin, but not for recognition of alpha (2)- macroglobulin, which binds to the avian homologue of ApoER2 harboring 8 lig and binding repeats.