Cysteine-rich Wnt-binding domain (CRD) of the frizzled 9 (Fz9) receptor.
The cysteine-rich domain (CRD) is an essential extracellular portion of the frizzled 9 (Fz9) receptor, and is required for binding Wnt proteins, which play fundamental roles in many aspects of early development, such as cell and tissue polarity, neural synapse formation, and the regulation of proliferation. Fz proteins serve as Wnt receptors for multiple signal transduction pathways, including both beta-catenin dependent and -independent cellular signaling, as well as the planar cell polarity pathway and Ca(2+) modulating signaling pathway. CRD containing Fzs have been found in diverse species from amoebas to mammals. 10 different frizzled proteins are found in vertebrata. Fz9 may play a signaling role in lymphoid development and maturation, particularly at points where B cells undergo self-renewal prior to further differentiation.