Plateins: a novel family of signal peptide-containing articulins in euplotid ciliates

In euplotid ciliates, the cortex is reinforced by alveolar plates--proteinaceous scales located within the membranous alveolar sacs, forming a monolayer just below the plasma membrane. This system appears to play a cytoskeletal role analogous to that provided by the fibrous epiplasm found beneath the cortical alveoli in other ciliates. In Euplotes aediculatus, the major alveolar plate proteins (termed alpha-, beta-, and gamma-plateins) have been identified. Using anti-platein antibodies, an expression library of Euplotes genes was screened, and a platein gene identified, cloned, and completely sequenced. Comparison of its derived amino acid sequence with microsequences obtained directly from purified plateins identified this gene as encoding one of the closely related beta- or gamma-plateins. The derived protein, of 644 amino acids (74.9 kDa), is very acidic (pI = 4.88). Microsequences from authentic alpha-platein were then used to design oligonucleotide primers, which yielded, via a PCR-based approach, the sequences of two alpha-platein genes from E. aediculatus. Even more acidic proteins, the derived alpha1- and alpha2-plateins contain 536 and 501 residues, respectively. Analyses of their amino acid sequences revealed the plateins to be members of the articulin superfamily of cytoskeletal proteins, first described in Euglena and now identified in the ciliate Pseudomicrothorax and in Plasmodium. The hallmark articulin repetitive motifs (based on degenerate valine- and proline-rich 12-mers) are present in all three plateins. In beta/gamma-platein this primary motif domain (27 repeats) is central in the molecule, whereas the primary repeats in the alpha-plateins lie near their C-termini. A cluster of proline-rich pentameric secondary repeats is found in the C-terminus of beta/gamma-platein, but near the N-terminus of alpha-plateins. All three plateins contain canonical N-terminal signal sequences, unique among known cytoskeletal proteins. The presence of start-transfer sequences correlates well with the final intra-alveolar location of these proteins. This feature, and significant differences from known articulins in amino acid usage and arrangement within the repeat domains, lead us to propose that the plateins comprise a new family of articulin-related proteins. Efforts to follow microscopically the assembly of plateins into new alveolar plates during pre-fission morphogenesis are underway.