Inositol polyphosphate 5-phosphatase OCRL: Difference between revisions

'''Lowe syndrome''', formally called oculocerebrorenal syndrome, oculocerebrorenal syndrome of Lowe or '''OCRL''', is an '''X-linked multisystemic disorder''' mainly involving eyes, nervous system (both the central and the peripheral) and kidneys and it is caused by '''mutations in OCRL1 protein'''. The syndrome is quite rare, its prevalence is 1 in 500 000 in the general population (based on the observations of the American Lowe Syndrome Association and the Italian Association of Lowe syndrome). Almost all of the patients are male. The syndrome is believed to occur worldwide as there are documented cases in America, Europe, Australia, Japan and India.<ref name="Lowe syndrome">PMID: 20301653</ref><ref name="Oculocerebrorenal">PMID: 27011217</ref>

The 901 amino acid long OCRL1 is composed of multiple domains which enable it to interact with various partners. OCRL1 consists of an N-terminus '''pleckstrin homology (PH)''' domain without a basic patch required for phosphoinositide recognition and binding. On the other hand, it contains a loop outside of the domain fold that is involved in OCRL1 recruitment to endocytic clathrin-coated pits. <ref name="PH">PMID: 19536138</ref>  

PH domain is followed by one of the major conserved domains of OCRL1 which is a central '''5-phosphatase (5P) domain''', in which two characteristic motifs are present (WXGDXN(F/Y)R and P(A/S)W(C/T)DRIL separated by 60-75 amino acids (AAs)). These play an important role in both substrate binding and catalysis.<ref name="OCRL">PMID: 16101675</ref> This domain has a Dnase I-like fold. <ref name="IP5">PMID: 22381590</ref>

Next up is an '''ASPM-SPD-2-Hydin (ASH)''' domain composed of nine β-strands forming two layers and a small α-helix. The β-sheet structure is similar to the immunoglobulin G fold. ASH domain also includes a Rab-binding site which mediates the interaction of OCRL1 with Rab-GTPases. This interaction is crucial for targeting OCRL1 to the Golgi complex and endosomal membranes.<ref name="mut">PMID: 33139981</ref>

At a region towards the C-terminus there is a catalytically inactive '''Rho-GAP-like domain'''. It shows homology to the Rho-GAP domain found in proteins that bind and stimulate the GTPase activity of the Rho family proteins. The two reasons why this domain has no GTPase stimulating activity are the replacement of the catalytic Arg by His and absence of a signle helix.<ref name="role">PMID: 17765681</ref> The ASH-RhoGAP module mediates the interactions of OCRL1 with proteins that promote specific targeting to various cellular destinations such as '''early endosomes''', '''Golgi complex''', '''lysosomes''' and '''primary cilium'''.<ref name="Lowe and Dent">PMID: 28669993</ref> Since these two function as a single folding module, a destabilization in one of them will affect the stability of the other. <ref name="role"/>

The membrane lipids phosphatidylinositol can be phosphorylated at positions 3, 4 and 5 of the inositol ring, which generates eight possible species called phosphoinositides. OCRL1 is one of the phosphatases that removes phosphate groups from specific positions of the inositol ring because it selectively '''acts as a 5-phosphatase'''. Phosphatidylinositols together with proteins of the Rab family typically have their distinct subcellular localization, and they are both an integral part of the recognition machinery of membrane compartments, which regulates membrane trafficking between organelles. Rab GTPases regulate membrane trafficking through interactions with various effectors, one of them being the phosphatase OCRL1.<ref name="main">PMID: 21378754</ref>

OCRL1 shows multiple binding sites for clathrin coat components (clathrin heavy chain and AP2 clathrin adaptor) so it seems to have a '''role in clathrin-mediated endocytosis''' because it is recruited to clathrin‐coated pits at the later stages of the vesicle formation process.<ref>PMID: 26351914</ref>. The two motifs involved in clathrin binding are located in the PH and Rho-GAP-like domains.<ref name="role"/> The 8 AA insertion in the longer isoform A enhances the interaction with clathrins.<ref name="isoform">PMID: 19211563</ref>

OCRL1 has been also reported to localize to the basal body and the transition zone of the primary cilium. Therefore, it also '''participates in ciliogenesis''' by contributing to protein trafficking to this organelle in an Rab8/IPIP27-dependent manner.<ref name="cilia">PMID: 22228094</ref>

Given the important functions of OCRL1 and the amount of its interaction partners it is not surprising that point mutations can cause the serious OCRL. Although, some mutations cause only a mild type of OCRL which is called '''Dent-2 disease'''.<ref name="com">PMID: 31967472</ref> This diseases is caused by different mutations in all domains of OCRL1 just like OCRL.<ref name="china">PMID: 31674016</ref><raf name="com"/><ref name="FH">PMID: 21666675</ref> However, it is characterized solely by heterogeneous kidney malfunctions.<ref name="dent">PMID: 32860533</ref> Even though certain continuum between the two diseases has been suggested it is unclear what causes the different symptoms of various mutations.<ref name="continum">PMID: 21031565</ref> As to the OCRL1 mutations causing OCRL so far only two have been studied closely. It is the substitution of F by V at the position 668 ('''F668V''') and the substitution of N by K at the position 591 ('''N591K''').<ref name="main"/><ref name="com"/>

It is clear from the structure that F668 is important in the binding site #2 because it sits in the <scene name='88/881644/Hydrophobic_pocket/1'>hydrophobic pocket</scene> of Rab8a created by I41, G42 and F70. Its substitution by V is therefore a major one since V is smaller and less hydrophobic than F. The mutation then causes disruption of this interaction and reduces the binding ability of OCRL1 with Rab8a by almost 6 folds. Moreover, the mutation causes the protein to be mainly localized in cytoplasm which can significantly hinder its normal function which is connected with vesicular formation.<ref name="main"/>

The N591K mutation also causes significant reduction in binding of Rab8a protein but the reason for this is different than in the case of F668V mutation. This AA is not part of any binding site but it seems to be important in the maintenance of the correct features of the ASH domain which are essential for the Rab8a binding. The effect of this mutation was studied in silico and it showed that the mutation causes the ASH domain to alter its flexibility and overall fold. Although the most significant change was observed in the AAs that surrounds the N591K mutation, the substitution caused subsequent changes in most parts of the protein which brought about decreases of prevalence of hydrogen bonds between Rab8a and OCRL1 which led to a lower stability of their interaction.<ref name="com"/>