Title : Membrane topology of the
ATP binding cassette transporter
ABCR and its relationship to
ABC1 and related ABCA transporters: identification of N-linked glycosylation
sites
Abstract :
- ABCR is a member of the ABCA subclass of ATP binding cassette transporters that is responsible for Stargardt macular disease and implicated in retinal transport across photoreceptor disc membranes
- It consists of a single polypeptide chain arranged in two tandem halves, each having a multi-spanning membrane domain followed by a nucleotide binding domain
- To delineate between several proposed membrane topological models, we have identified the exocytoplasmic (extracellular/lumen) N-linked glycosylation sites on ABCR
- Using trypsin digestion, site-directed mutagenesis, concanavalin A binding, and endoglycosidase digestion, we show that ABCR contains eight glycosylation sites
- Four sites reside in a 600-amino acid exocytoplasmic domain of the N-terminal half between the first transmembrane segment H1 and the first multi-spanning membrane domain , and four sites are in a 275-amino acid domain of the C half between transmembrane segment H7 and the second multi-spanning membrane domain
- This leads to a model in which each half has a transmembrane segment followed by a large exocytoplasmic domain , a multi-spanning membrane domain , and a nucleotide binding domain
- Other ABCA transporters, including ABC1 linked to Tangier disease, are proposed to have a similar membrane topology based on sequence similarity to ABCR
- Studies also suggest that the N and C halves of ABCR are linked through disulfide bonds