The Alp family of proteins. The Alp family of proteins. (A) Comparison of the four known members of the Alp protein family: the α, Rib, R28, and Alp2 proteins. S, signal peptide; N, nonrepeated N-terminal region; R, repeat region; C, C-terminal region. The number of amino acid residues in each region and the percent residue identity (bold numbers) between corresponding regions are indicated. Note that the designation Alp3 has been used as an alternative name (130) for the R28 protein (235). Each of the three proteins α, Rib, and R28 has 9 to 12 repeats with a length of 79 or 82 residues, as indicated. These repeats are identical within one protein but not between proteins, and the number of repeats varies among clinical isolates. The nonrepeated region of R28 contains a 195-residue subregion, designated RN, which does not fit into the alignment with Rib. This region is 99% identical to a type of repeat found in the N-terminal half of the Alp2 protein. Thus, the Alp2 protein contains two types of tandem repeat (designated RN and R), unlike the other members of the family. This figure is based on published sequences for the four proteins (78, 130, 165, 235, 261). The data for Alp2 are based on the sequence reported by Glaser et al. (78). (B) Schematic representation of the R28 protein. As indicated, R28 can be viewed as a chimera, derived from the two Alp family proteins α and Rib and the unrelated β protein (235). Predictions about the three-dimensional structure are given below the protein. The region that shows similarity to β corresponds to a region in β predicted to have an IgSF fold (12), and our analysis of the R28 region suggests that it has a similar structure. The repeats in the C-terminal part of R28 have been proposed to have a fold related to the IgSF fold (35). Modified from reference 235 with permission of the publisher. (C) Immunological relationship among the four known members of the Alp protein family. Solid arrows indicate immunological cross-reactivity, and broken arrows indicate lack of cross-reactivity. For example, the α protein cross-reacts with Alp2 but not with Rib or R28. Most of the data summarized in the figure were obtained with rabbit antisera raised against purified proteins (132, 234-236), but the relationship between R28 and Alp2 was analyzed with an extract of strain D136C (NEM316), which expresses Alp2 (78, 234). (D) Laddering pattern in the α, Rib, and R28 proteins analyzed by SDS-PAGE. Purified preparations of the three proteins, and the control protein β (which is not a member of the Alp family), were subjected to SDS-PAGE under ordinary conditions (with boiling of samples at pH = 7) or after boiling of samples for 15 min at pH = 4. Under the latter conditions, the three Alp family proteins form a regularly laddered pattern due to hydrolysis of acid-sensitive Asp-Pro bonds in the repeats during sample preparation. Modified from reference 235 with permission of the publisher. Gunnar Lindahl et al. Clin. Microbiol. Rev. 2005; doi:10.1128/CMR.18.1.102-127.2005