Characterization of a 29.4-kilodalton structural protein of Giardia lamblia and localization to the ventral disk [corrected].

The amino acid sequence of a 29.4-kilodalton [corrected] structural protein located in the ventral disk and axostyle of Giardia lamblia was determined. Clone lambda M16 from a mung bean expression library in lambda gt11 expressed a fusion protein recognized by three different isolate-specific antisera and sera from G. lamblia-infected gerbils. One of the three EcoRI fragments (M16; 1.26 kilobases) encoded the recognized protein. Sequence analysis revealed a single open reading frame of 813 base pairs. Two areas showed conservation of the positions of some amino acids. The abundance of arginine, glutamic acid, and threonine was increased. Two potential alpha-helical regions were deduced in the regions of repeats. Antisera to the M16 fusion protein reacted specifically with internal components of the ventral disk and axostyle, as well as Giardia fractions enriched for ventral disk structural proteins. An identical protein was recognized in different isolates by anti-M16, and a single identical band was recognized in Southern blots using the M16 1.26-kilobase fragment as a probe. Therefore, the 29.4-kilodaltion [corrected] protein appears to be highly conserved compared with variant surface proteins.