Phototropism of Arabidopsis thaliana seedlings in response to a blue light source is initiated by nonphototropic hypocotyl 1 (NPH1), a light-activated serine-threonine protein kinase. Mutations in NPH3 disrupt early signaling occurring downstream of the NPH1 photoreceptor. The NPH3 gene encodes a NPH1-interacting protein. NPH3 is a member of a large protein family, apparently specific to higher plants, and may function as an adapter or scaffold protein to bring together the enzymatic components of a NPH1-activated phosphorelay [1].

1: Arabidopsis NPH3: A NPH1 photoreceptor-interacting protein essential for phototropism. Motchoulski A, Liscum E; Science 1999;286:961-964. PMID:10542152