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