C-jun-amino-terminal kinase-interacting protein 4 is an enzyme that in humans is encoded by the SPAG9 gene.

Extracellular signals are transduced into cells through mitogen-activated protein kinases. The structural organization of these kinases into specific signaling domains is facilitated by scaffolding proteins involved in closely tethering different kinases so that successive phosphorylation events can occur. The protein encoded by this gene is a scaffolding protein that brings together mitogen-activated protein kinases and their transcription factor targets for the activation of specific signaling pathways. This gene which is abundantly expressed in testicular haploid germ cells encodes a protein that is recognized by sperm-agglutinating antibodies and implicated in infertility. Alternatively spliced transcript variants encoding different isoforms have been found for this gene. [provided by RefSeq, Jul 2008].
Function: The JNK-interacting protein (JIP) group of scaffold proteins selectively mediates JNK signaling by aggregating specific components of the MAPK cascade to form a functional JNK signaling module. Isoform 5 may play a role in spermatozoa-egg-interaction.