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2.A.22.6.3
B(O)AT1 or BOAT (SLC6A19; Hartnup's disease protein) is a kidney and intestinal apical membrane epithelial transporter for Na+-dependent, Cl--independent reabsorption of neutral amino acids. Many neutral L-amino acids bind with ~0.5 mM affinities; Leu is the preferred substrate, but all large, neutral, non-aromatic, L-amino acids bind to this transporter. Uptake of leucine is sodium-dependent. In contrast to other members of the neurotransmitter transporter family, this one does not appear to be chloride-dependent.  Activity is enhanced by collectrin (Tmem27), a collecting duct transmembrane (1 TMS) glycoprotein (Q9HBJ8) (Danilczyk et al., 2006). The mouse orthologue is (Q9D687) (Broer et al., 2004; 2008) which is deficient due to mutation(s) in its structural gene, and it forms a complex with collectrin and the brush border carboxypeptidase angiotensin-converting enzyme 2 (ACE2; Q9BYF1). Mutations in Hartnup disorder protein, such as B0AT1(R240Q), decrease complex formation (Kraut and Sachs 2005) and lead to neutral aminoaciduria and in some cases pellagra-like symptoms (Kowalczuk et al., 2008; Singer et al. 2012).  Collectrin is expressed at high levels in the simple embryonic kidney (the pronephros) of amphibians such as Xenopus  (McCoy et al. 2008). ACE2 plays an important role in amino acid transport by acting as a binding partner of SLC6A19 in  the intestine, regulating its trafficking, expression on the cell surface and catalytic activity (Kowalczuk et al. 2008, Camargo et al. 2009). ACE2 is also the cellular receptor for SARS-CoV and SARS-CoV-2 ( causitive agent of COVID-19). Yan et al. 2020 presented cryoEM structures of full-length human ACE2 in the presence of B0AT1 with or without the receptor binding domain (RBD) of the surface spike glycoprotein (S protein) of SARS-CoV-2, both at an overall resolution of 2.9 angstroms. The ACE2-B0AT1 complex is assembled as a dimer of heterodimers, with the collectrin-like domain of ACE2 mediating homodimerization. The RBD is recognized by the extracellular peptidase domain of ACE2 mainly through polar residues (Yan et al. 2020).

Accession Number:Q9HBJ8
Protein Name:Transmembrane protein 27 aka TMEM27
Length:222
Molecular Weight:25235.00
Species:Homo sapiens (Human) [9606]
Number of TMSs:1
Location1 / Topology2 / Orientation3: Membrane1 / Single-pass type I membrane protein2
Substrate Na+, Cl-, Neutral amino acids

Cross database links:

RefSeq: NP_065716.1   
Entrez Gene ID: 57393   
OMIM: 300631  gene
KEGG: hsa:57393   

Gene Ontology

GO:0016021 C:integral to membrane
GO:0008237 F:metallopeptidase activity
GO:0008241 F:peptidyl-dipeptidase activity
GO:0006508 P:proteolysis

References (5)

[1] “Collectrin, a collecting duct-specific transmembrane glycoprotein, is a novel homolog of ACE2 and is developmentally regulated in embryonic kidneys.”  Zhang H.et.al.   11278314
[2] “The secreted protein discovery initiative (SPDI), a large-scale effort to identify novel human secreted and transmembrane proteins: a bioinformatics assessment.”  Clark H.F.et.al.   12975309
[3] “Complete sequencing and characterization of 21,243 full-length human cDNAs.”  Ota T.et.al.   14702039
[4] “The status, quality, and expansion of the NIH full-length cDNA project: the Mammalian Gene Collection (MGC).”  The MGC Project Teamet.al.   15489334
[5] “The HNF-1 target collectrin controls insulin exocytosis by SNARE complex formation.”  Fukui K.et.al.   16330323

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FASTA formatted sequence
1:	MLWLLFFLVT AIHAELCQPG AENAFKVRLS IRTALGDKAY AWDTNEEYLF KAMVAFSMRK 
61:	VPNREATEIS HVLLCNVTQR VSFWFVVTDP SKNHTLPAVE VQSAIRMNKN RINNAFFLND 
121:	QTLEFLKIPS TLAPPMDPSV PIWIIIFGVI FCIIIVAIAL LILSGIWQRR RKNKEPSEVD 
181:	DAEDKCENMI TIENGIPSDP LDMKGGHIND AFMTEDERLT PL