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. May function as a regulator of vesicle transport, through interactions with the JNK-signaling components and motor proteins (PubMed:10523642, PubMed:10629060). Promotes neuronal axon elongation in a kinesin- and JNK-dependent manner (PubMed:23576431, PubMed:25944905, PubMed:28259553). Activates cofilin at axon tips via local activation of JNK, thereby regulating filopodial dynamics and enhancing axon elongation (PubMed:23576431, PubMed:25944905, PubMed:28259553). Its binding to kinesin heavy chains (KHC), promotes kinesin-1 motility along microtubules and is essential for axon elongation and regeneration (PubMed:23576431, PubMed:25944905, PubMed:28259553). Regulates cortical neuronal migration by mediating NTRK2/TRKB anterograde axonal transport during brain development (PubMed:23576431, PubMed:25944905, PubMed:28259553). Acts as an adapter that bridges the interaction between NTRK2/TRKB and KLC1 and drives NTRK2/TRKB axonal but not dendritic anterograde transport, which is essential for subsequent BDNF-triggered signaling and filopodia formation (PubMed:23576431, PubMed:25944905, PubMed:28259553). Forms homo- or heterooligomeric complexes. The central region of MAPK8IP3 interacts with the C-terminal of MAPK8IP2 but not MAPK8IP1. Binds specific components of the JNK signaling pathway namely MAPK8/JNK1, MAPK9/JNK2 and MAPK10/JNK3 to the N-terminal region, MAP2K4/MKK4 and MAP2K7/MKK7 to the central region and MAP3K11 to the C-terminal region. Binds the TPR motif-containing C-terminal of kinesin light chain, KLC1. Pre-assembled MAPK8IP1 scaffolding complexes are then transported as a cargo of kinesin, to the required subcellular location (PubMed:10523642, PubMed:10629060, PubMed:11238452, PubMed:11106729). Interacts with ROCK1 and this interaction is enhanced by ultraviolet-B (UVB) radiation. Interacts with SH3RF2 (By similarity). Interacts with NTRK2/TRKB and NTRK3/TRKC (PubMed:21775604). Localized in the soma and growth cones of differentiated neurites and the Golgi and vesicles of the early secretory compartment of epithelial cells (PubMed:10629060, PubMed:11106729). KIF5A/B/C-mediated transportation to axon tips is essential for its function in enhancing neuronal axon elongation (By similarity). Highly expressed throughout many regions of the brain and at lower levels in the heart, liver, lung, testes and kidney. All isoforms have been identified in the brain, isoform 1a is also expressed in the spleen and lung. Highly expressed in the brain of embryonic mice. Expression levels gradually increase from 10 dpc to postnatal day 10 (P10) (at protein level). Expressed in neurites 5 days following initiation of nerve growth factor induced differentiation. NGF withdrawal results in the down-regulation of MAPK8IP3 protein by caspase-mediated cleavage. Phosphorylation by ROCK1 is crucial for the recruitment of JNK. The cerebral cortex in the embryonic brain is a little thinner and the distribution of neurons is more disordered than that found in the wild-type littermates. Belongs to the JIP scaffold family.