Expression and sequence of Penumbra (E6-3). (A) Expression of E6-3 in a panel of cell lines used in the initial screening: NIH3T3 (fibroblasts), J774 (macrophages), MPRO (promyelocytes), EML C.1 (multipotent), MC/9 (mast cells), FLDS-19 MEL (erythroblasts/proerythroblasts), BaF3 (B-cell–like), EL4 (T-cell–like). Expression in uninduced EML C.1 is due to spontaneously generated proerythroblasts and erythroblasts. The ethidium bromide–stained gel is shown on the bottom. (B) Expression of E6-3 during erythroid differentiation. Lanes 1 to 8: EML C.1 stimulated with SCF+EPO for 0 to 7 days; lanes 9 and 10: EML C.1 stimulated with SCF+EPO for 7 days and then EPO alone for 1 and 2 days, respectively; lane 11: uninduced FLDS-19 MEL. The blot was sequentially hybridized with the E6-3 and mouse β^major-globin probes. (C) Amino acid sequences of mpen and hpen. Only differing aa's are shown for hpen. The 4 hydrophobic segments are underlined. Polar amino acids within the transmembrane segments are in bold. Asterisks denote conserved cysteine-containing motifs. (D) A hydrophilicity plot of mpen. (E) Amino acid sequence alignment between mpen and mCD63 by CLUSTAL W. “+” indicates identical aa; “:” indicates conserved substitution; “.” indicates semiconserved substitution.