MULTIPLE FORMS OF FIMBRIAE ON THE SPORIDIA OF CORN SMUT, USTILAGO-MAYDIS

At least three types of surface fibrils or fimbriae were observed in shadowed and negatively stained sporidia of Ustilago maydis examined with the electron microscope. ''Type A'' fimbriae were more than 10 mum in length and 7 nm in diameter; ''type B'' fimbriae were similar in length but narrower (3-4 nm); and ''type C'' fimbriae were also narrow at 3-4 nm but were short (< 1 mum) and frequently aggregated into loose ''cables'' containing two or three fibrils. SDS-PAGE analysis of the surface proteins showed four major bands with molecular masses of 76, 72, 50, and 31 kDa. The three bands of higher molecular mass gave positive results in Western blots when immunoprobed with polyclonal antiserum against the fimbriae from Microbotryum violaceum. The 31-kDa band was not probed. Similarly, an antiserum against the 31-kDa protein of U. maydis did not react with the 74-kDa protein of M. violaceum or the 76-, 72-, and 50-kDa proteins of U. maydis. All four bands were positive indicating glycoproteins; the 31-kDa band was weakly stained. Treatment with trifluoromethanesulfonic acid, which removed carbohydrate units, resulted in the replacement of these proteins by three new bands with molecular masses of 66, 47, and 30 kDa. The 66-kDa band may be a common protein moiety in the 76- and 72-kDa glycoproteins, the 47-kDa band the core protein of the 50-kDa glycoprotein, and the 30-kDa band the core protein portion of the 31-kDa glycoprotein. All four glycoprotein bands assembled spontaneously into fibrils following electroelution from the gel and dialysis. Morphological comparisons of the reassembled and intact fibrils on cells of U. maydis showed the fibrils formed by the 50-kDa band to be very similar to type A fibrils, while the 76- and 72-kDa bands formed fibrils that were similar to type B fimbriae, and the fibrils formed by the 31-kDa band resembled type C fimbriae. All isolates (including a1b1, a2b2, and a diploid) of a second strain of U. maydis produced fimbrial proteins of 76, 72, and 31 kDa but lacked the 50-kDa fimbrial protein. These strains produced fimbriae of types B and C but lacked the type A form.