ROCK1

ROCK1 is also the name of the gene that encodes the protein ROCK1, a serine/threonine kinase. ROCK1 is activated when bound to the GTP-bound form of RhoA. The human ROCK1 gene is located on human chromosome 18 with specific location of 18q11.1.[6] The location of the base pair starts at 18,529,703 and ends at 18,691,812 bp and translates into 1354 amino acids.[7]

ROCK1 has a ubiquitous tissue distribution, but subcellularly it is thought to colocalize with the centrosomes. This is consistent with its function as a key modulator of cell motility, tumor cell invasion, and actin cytoskeleton organization.[7] In rats, ROCK1 is expressed in the lung, liver, spleen, kidney, and testis.[8][9][10]

The ROCK1 structure is a serine/threonine kinase with molecular weight of 158 kDa.[7] It is a homodimer composed of a catalytic kinase domain (residues76-338)[11] located at the amino or N-terminus of the protein, a coiled-coil region (residues 425-1100)[11] containing the Rho-binding domain, and a pleckstrin-homology domain (residues 1118-1317)[11] with a cysteine-rich domain. When a substrate is absent, ROCK1 is an autoinhibited loop structure. Enzyme activity of ROCK1 is inhibited when the pleckstrin-homology and Rho-binding domains in the C-terminus independently bind to the N-terminus kinase domain. When a substrate such as GTP-bound RhoA binds to the Rho-binding region of the coiled-coil domain, the interactions between the N-terminus and the C-terminus are disrupted, thus activating the protein. Cleavage of the C-terminal inhibitory domain by caspase-3 during apoptosis can also activate the kinase.[12]

This view of autoinhibition released by RhoA binding has been challenged by low resolution electron microscopy data showing ROCK to be a constitutive linear dimer 120 nm in length.[13] According to this new data ROCK does not need to be activated by RhoA or phosphorylation because it is always active, and whether ROCK will phosphorylate its substrates (e.g. myosin regulatory light chain) depends only on their subcellular localization.[13]

There is one other isoform of ROCK known as ROCK2. ROCK2 is located at 2p24 and is highly homologous with ROCK1 with an overall amino acid sequence identity of 65%.[11] The identity in the Rho-binding domain is 58%[11] and approximately 92%[11] in the kinase domain. The ROCK isoforms are encoded by two different identified genes and are ubiquitously expressed.[11]

GTPase-RhoA binding can increase the activity of ROCK1 by 1.5-2-fold.[14] Without RhoA binding, lipids such as arachidonic acid or sphingosine phosphorylcholine can increase ROCK1 activity 5- to 6-fold.[14][15] These two lipids interact with the pleckstrin-homology domain, thus disrupting its ability to inhibit ROCK1.[16] G-protein RhoE binds to the N-terminus of ROCK1 and inhibits its activity by preventing RhoA binding. Small G-proteins, Gem and Rad, have been shown to bind and inhibit ROCK1 function, but their mechanism of action is unclear.[11]

ROCK1 phosphorylation sites are at RXXS/T or RXS/T.[11] More than 15 ROCK1 substrates have been identified and activation from these substrates most often leads to actin filament formation and cytoskeleton rearrangements.[11] MYPT-1 is involved in a pathway for smooth muscle contraction. When ROCK1 is activated by binding of GTPase RhoA it produces multiple signaling cascades. For example, RhoA is one of the downstream signaling cascades activated by vascular endothelial growth factor (VEGF). ROCK1 acts as a negative regulator of VEGF endothelial cell activation and angiogensis.[17] ROCK1 activation by RhoA also promotes stabilization of F-actin, phosphorylation of regulatory myosin light chain (MLC) and an increase in contractility, which plays a crucial role in tumor cell migration and metastasis.[18] This activated ROCK1 also activates LIM kinase, which, phosphorylates cofilin, inhibiting its actin-depolymerizing activity.[19] This depolymerization results in stabilization of actin filaments and decreased branching which promotes contraction.

Cardiac troponin is another ROCK1 substrate that upon phosphorylation causes reduction in tension in cardiac myocytes.[11] ROCK1 also acts as a suppressor of inflammatory cell migration by regulating PTEN phosphorylation and stability.

ROCK1 has a diverse range of functions in the body. It is a key regulator of actin-myosin contraction, stability, and cell polarity.[17] These contribute to many progresses such as regulation of morphology, gene transcription, proliferation, differentiation, apoptosis and oncogenic transformation.[5] Other functions involve smooth muscle contraction, actin cytoskeleton organization, stress fiber and focal adhesion formation, neurite retraction, cell adhesion and motility. These functions are activated by phosphorylation of DAPK3, GFAP, LIMK1, LIMK2, MYL9/MLC2, PFN1 and PPP1R12A.[17] Additionally, ROCK1 phosphorylates FHOD1 and acts synergistically with it to promote SRC-dependent non-apoptotic plasma membrane blebbing.[17] It is also required for centrosome positioning and centrosome-dependent exit from mitosis.[17]

ROCK1 has been shown to interact with:

• LIMK1,[5]
• MLC,[5]
• MYPT1,[5]
• PFN2,[20] and
• RHOA.[21][22][23]

In humans, the main function of ROCK1 is actomyosin contractility. As mentioned before, this contributes to many proximal progresses such as regulation of morphology, motility, and cell–cell and cell–matrix adhesion.[5] In addition, ROCK kinases influence more distal cellular processes including gene transcription, proliferation, differentiation, apoptosis and oncogenic transformation.[5] Given this diverse range of functions, it is not surprising that ROCK1 has been implicated in numerous aspects of cancer.[5]

• Rho kinase

• ROCK1 Info with links in the Cell Migration Gateway