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Fig. 4. CR-containing proteins and their possible roles in positive feedback and
spatial bi-stability. (A) The domain structures of several
BMP-binding proteins. Sog is a secreted (signal peptide, SP, yellow) protein
that contains BMP-binding modules [cysteine-rich motifs (CRs), green], whereas
Cv-2, and its vertebrate homologs (not shown), and other related vertebrate
proteins, such as Kielin or KCP, contain both CR domains and a von Willebrand
Factor D (VWFD) motif (red) that may promote cell surface localization. The
CRIM1 protein also contains CR domains but instead of a VWFD, it contains an
insulin-like growth factor binding protein domain (IGFBP), a transmembrane
domain (TM) that is likely to anchor it to the cell surface, and a small
cytoplasmic tail (Cyto). (B) Position (or BMP) versus BMP-bound
receptor for on/off equilibrium (dotted line) and positive feedback induced
bi-stability (solid line). In general the distribution of morphogen is
non-linear in x [BMP=f(x)]; however, in this case, the
extracellular gradient of BMP is linear in position (i.e.
BMP
*x). As the level of BMP increases, the
level of BMP-bound receptor follows the on/off equilibrium solution until it
reaches a limit point (LP) where the lower equilibrium solution ceases to
exist. For levels of BMP above this point, the level of BMP-bound receptor
approaches the upper stable branch.
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