Family with Sequence Similarity 155 Member B is a protein in humans that is encoded by the FAM155B gene. It belongs to a family of proteins whose function is not yet well understood by the scientific community. It is a transmembrane protein that is highly expressed in the heart, thyroid, and brain.
Gene
FAM155B is located on the X chromosome at the position Xq13.1. It is found on the positive strand from nucleotides 69504326-69532508. Genetic neighbors include the gene EDA (ectodysplasin A) downstream and a long intergenic non-protein coding RNA upstream. The full gene transcript sequence of this gene is 3528 bp long with a coding sequence of 1418 bp. This transcript includes 3 exons and 2 introns.
Aliases of FAM155B are TMEM28, cXorf63, and TED.
mRNA
The FAM155B gene has 2 known isoforms.
The transcript of isoform 1 is 4685 bp long with a coding sequence of 1022 bp. It has a 5' UTR consisting of 79 bp and a 3' UTR with 3583 bp.
The transcript of isoform 2 is much shorter at 975 bp with a coding sequence of 878 bp. It also has a 5' UTR of 79 bp, but the 3' UTR is simply 17 bp.
Protein
Composition
The protein sequence of FAM155B is 472 amino acids long. The molecular weight is predicted to be 52.5 kDa and the isoelectric point is estimated to be 8.2. The most prominent amino acids are Leucine (L) and Proline (P) as they account for 11.4% and 10% of the protein respectively. In terms of amino acid grouping, the most common ones are AGP, LVIFM, and KRED at 24.8%, 22.2%, and 21%. This protein lacks charged segments or charge clusters.
There are 2 protein isoforms for FAM155B. Isoform 1 is 340 amino acids long while isoform 2 is composed of 292 amino acids.
Domains
FAM155B has two transmembrane domains which suggests that it is a multi-pass membrane protein. It also has a cytoplasmic domain located right between the two transmembrane domains and three compositionally biased regions. A cysteine-rich region is located at the beginning of the sequence followed by a proline-rich region and a histidine-rich region much later in the sequence.
Secondary structure
This protein is predicted to have 9 beta sheets, 16 alpha helices, and 2 transmembrane helices.
Tertiary structure
In terms of membrane topology, the N- and C-termini appear to be located extracellularly while the protein sequence between the transmembrane domains appears to be located in the cytoplasmic region.
Regulation
DNA-level regulation
The only regulatory element is a promoter region that is located about 2303 bp upstream from the transcriptional start site. There is one DNAse hypersensitivity cluster and one CpG island associated with the promoter region. There is low signal strength among cell lines among both H3K4me3 and H3K27Ac tracks. The H3k4me1 track exhibit relatively higher signals in the NHEK and H1-hESC cell lines. Multiple transcription factors binding sites are associated with the promoter such as ERE, C2H2, and TFIIB.
RNA-level regulation
There are no known miRNA target sites in FAM155B. Many stem-loop structures are predicted from the 3' UTR of human FAM155B and close orthologs. This indicates some conservation of secondary mRNA structures among different species.
Protein-level regulation
Localization
Human FAM155B along with closely related orthologs are most likely to be localized in the endoplasmic reticulum with a prediction of 34.8%. Following this, the protein is likely to be found in the plasma membrane with a prediction of 21.7%.
Post-translational modification
FAM155B has two proposed N-Glycosylation sites at Asparagine 120 and 193. There do not appear to be any significant O-Glycosylation sites. There are two CKII phosphorylation sites at Threonine 302 and 469 along with two PKC phosphorylation sites at 283 and 349. A sumoylation analysis identified two motifs with low probability scores of 0.5 and 0.13. Since the probability scores are low, this indicates that FAM155B is unlikely to have any true sumoylation motifs.
Expression
Highest expression is observed in the heart, thyroid, and brain tissues from RNA sequencing of human tissues. Specifically, fetal brain tissue and the cerebellum are observed to have higher expression than the brain as a whole. In regards to fetal tissue, high expression of the heart was observed at 11 weeks while the stomach and intestine are expressed highly at 20 weeks. The kidney is also expressed relatively highly at 10 weeks. This implies that expression in tissues like the stomach, intestine, and kidneys decreases as fetal development continues.