Characterization of a proteasome alpha-chain from Giardia lamblia

To begin to characterize the components of the 20S proteasome of Giardia la mblia, we have cloned a genomic sequence encoding an alpha -chain (type alp ha3/C9, predicted size 244 amino acid residues). Southern analysis indicate d that a single gene codes for this protein, and a Northern blot exhibited a single signal at 850 nt. An antiserum against a C-terminal fragment of th e alpha -chain expressed in Escherichia coli reacted with a single protein band of M-r 27,000 that was present at constant levels in trophozoites and encysting cells. On a 2D blot of the purified 20S proteasome, we identified the crossreacting component as a single protein of IEP 6.0, in agreement w ith the IEP predicted by the coding sequence. Our data confirm that the G. lamblia 20S proteasome is typically eukaryotic in containing a set of diver ged alpha -subunits.