SDS-PAGE analysis of protein 4.1 in age-defined dog RBC membranes. RBCs were isolated from dog no. 1630 or dog no. 5005 at the indicated times post-biotinylation by magnetic cell sorting and ghost membranes were prepared as described in Materials and Methods. Equivalent amounts (20 μg) of membrane protein were electrophoresed on gradient (6% to 12%) polyacrylamide gels and stained for protein with Coomassie brilliant blue. (A) Membranes from nonbiotinylated cells retrieved from dog no. 1630; (B) membranes from biotinylated cells retrieved from dog no. 1630; (C) membranes from biotinylated cells retrieved from dog no. 5005. C denotes control cells not subjected to biotin-avidin sorting.