The extracellular matrix plays an integral role in the pivotal processes of development,
tissue repair, and metastasis by regulating cell proliferation, differentiation, adhesion,
and migration. This review is focused on a family of related glycoproteins represented
by at least one member in all specialized extracellular matrices. This family currently
comprises nine members grouped together on the basis of their presence in the extracellular
matrix and by virtue of a leucine-rich repeat motif that dominates the structure of
the core protein. It is likely that most, if not all the members of this group exist
as proteoglycans in some tissues, and thus have been termed the Small Leucine-Rich
Proteoglycan family, or SLRPs. The leucine-rich repeat (LRR) is usually present in
tandem array and has been described in an increasing number of proteins, giving rise
to a LRR-superfamily. The LRR domain of the SLRP family is unique within the superfamily
in that it is flanked by cysteine clusters, and the 24 amino acid consensus for SLRP
members is x-x-I/V/L-x-x-x-x-F/P/L-x-x-L/P-x-x-L-x-x-L/I-x-L-x-x-N-x-I/L, where x
is any amino acid. Enormous progress has been made in describing the membership, structure
and localization of this family, and recently new insight has emerged into the putative
function of these molecules not just as modulators of matrix assembly but also on
their intriguing role in regulating cell growth, adhesion, and migration. Determination
of membership, structure and putative function of this fascinating class of molecules
is summarized in this review.