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Fig. 1. Sequence alignments of L. monocytogenes internalin LRR regions. Stars above the sequences denote conserved internalin LRR residues, and bars denote the position of β-strands and extent of 310-helices observed in the InlB structure (39). Residues predicted to form the concave or convex face of these structures are enclosed in boxes. Hydrophobic, negatively charged, and positively charged residues predicted to be surface exposed are highlighted in yellow, red, and cyan, respectively. Sequences for InlA, InlB, InlC, InlC2, InlD, InlE, InlF, InlG, and InlH are shown. The repeats are 22 residues in length except for repeat 4 of InlC and repeat 6 of InlA, which are 21 residues, and repeat 5 of InlF, which is 23 residues. These small deletions and insertions are predicted to occur at loop regions.

U2LRR fragment of U2 small nuclear ribonucleoprotein ( 43), and the Ran GTP-ase activating protein rnalp (44). As in these structures, each repeat alternates between a short β-strand and an opposing antiparallel helical segment; the β-strands and helices are connected to each other by coils. The main chain wraps around in a right-handed sense, with the β-strands forming a parallel β-sheet and the helices stacking on each other, giving rise to the elongated shape of the LRR.

Although the β-strands are a highly conserved structural feature of LRR proteins, the helical segments are more divergent. The β-strands are almost always composed of three residues that are in precise register. On the other hand, the helical segments are variable in length, register, and type. In fact, one repeat of U2LRR contains a second β-strand instead of a helix. In InlB, the helical segments are short (three to five residues) and are exclusively composed of 310-helices. U2LRR possesses 23-and 25-residue repeats that also form 310-helices. This contrasts with RI and rnalp, which have long (10–14 residues) α-helices and also have longer repeat lengths (28 residues or greater). Apparently the more tightly wound 310-helix accommodates formation of short repeats more favorably than does the α-helix.

The curvature of the LRR region arises from the β-strands of adjacent repeats being packed more closely together than opposing helices. The β-strands are within interrepeat hydrogen bonding distance, whereas the helices form intrarepeat hydrogen bonds. The β-strands form the concave face and the helices the

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