RAB27

Rab27 is a member of the Rab subfamily of GTPases. Rab27 is post translationally modified by the addition of two geranylgeranyl groups on the two C-terminal cysteines.

RAB27A, member RAS oncogene family
Identifiers
Symbol	RAB27A
NCBI gene	5873
HGNC	9766
OMIM	603868
RefSeq	NM_004580
UniProt	P51159
Other data
Locus	Chr. 15 q21
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RAB27B, member RAS oncogene family
Identifiers
Symbol	RAB27B
NCBI gene	5874
HGNC	9767
OMIM	603869
RefSeq	NM_004163
UniProt	O00194
Other data
Locus	Chr. 18 q21.2
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Isoforms

Rab27 has two main isoforms: Rab27a and Rab27b. These are similar in primary composition, with 66% similarity between nucleotides.^[1] Most of their differences originate from their C-terminal, which is responsible for interactions with proteins such as SPIREs.^[1][2] Thus, these isoforms play different roles in the regulation pathway of exocytosis. Throughout the process of exocytosis, Rab27a and Rab27b are found in different sections of the cell, with Rab27b found commonly in the TGN and Rab27a usually bound to multivesicular endosomes with CD63 present.^[3]

Function

Rab27 plays a key role in the regulation of exocytosis of vesicles in various cellular organelles.^[4] Rab27 uses effectors to tether vesicles to itself and transport them to the plasma membrane, where they undergo fusion.^[4] They ensure that vesicles attach correctly, in the proper orientation, at the dedicated site of fusion. However, fusion itself is started when effectors bind SNARE proteins that catalyze the start of exocytosis.^[4]

Clinical significance

Mutations

Mutations that prevent the expression of Rab27 ('knock out' mutations) cause the hypopigmentation and immunodeficiency disorder known as type II Griscelli syndrome, while a decrease in Rab27 prenylation is thought to be involved in choroideremia.

The symptoms of type II Griscelli syndrome have shown that Rab27 is involved in melanosome transport in melanocytes and in cytotoxic killing activity in cytotoxic T lymphoblasts. In melanocytes Rab27 binds the melanosome. The melanosome is transported along the microtubule. Rab27 then recruits Slac2A and myosin Va, these enzymes are essential for the transfer of the melanosomes from the microtubules to actin filaments. The melanosomes can now continue on their path towards the cell periphery. If either Rab27, Slac2A or myosin Va are absent then the melanosomes remain in the perinuclear region of the cell. This disruption in pigmentation results in the hypopigmentation seen in the silvery hair colour of patients with Griscelli syndrome.

Cancer

The Rab27a isoform might play a role in cancer. It contributes to the growth of cancerous tumors due to its promotion of chemokine and metalloproteinase secretions.^[5] Because of the over-expression of Rab27a in tissues in the breasts, lungs, and pancreas, there may be a linkage between Rab27a presence and likelihood of cancer.^[5] Additionally, the release of metalloproteinases results in the breakdown of the ECM, which releases many of the growth factors held within.^[6]