Proline and Polyproline Type II Helices

The SH3 Domain A Model System to Understand Interactions Mediated by Proline-Rich PPII Helices 

The almost concomitant report of the crystal and solution structures from several unliganded SH3 domains (31-36) revealed a completely conserved fold a compact p-barrel, formed by two anti-paraUel, three-stranded p-sheets (Fig. 2A). A structure-based alignment of a subset of SH3 domains is shown in Fig. 2B. The structure starts at the poorly conserved pi strand. After this strand, the polypeptide chain engages in the formation of the so-called RT loop, connecting strands pA and pB (see below for an explanation of this nomenclature). One of the sequence 

The SH3 fold has been observed in other protein domains with distinct function (relevant information can be found in reference 37). Recently, a group of bacterial proteins bearing significant sequence similarity to the SH3 domain was discovered (38). Whether these prokaryotic domains share the SH3 fold and are involved in PPII recognition is currently unknown. The simplicity of the fold, the great wealth of structural information, and the ease of its biochemical handling have made the SH3 domain a very-well-characterized model system for the study of proteinfolding mechanisms (see for instance references 39-46), but these studies will not be reviewed here. 

Experimentally Determined Three-Dimensional Structures of SH3 Domains 

Complexes with peptides, peptoids, or nonpeptide elements  

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