2
$\begingroup$

Do histones contribute more (by mass) than non-histone proteins in the chromosomes formed during mitosis?

$\endgroup$
  • $\begingroup$ I restructured your question in clear direct English, but wonder why this information is important to you. $\endgroup$ – David May 1 at 12:24
2
$\begingroup$

If you are asking if histone mass represents a larger percentage of total chromosome mass then the answer is yes when considered at the level of the nucleosome. Each histone-octamer wraps ~147 base pairs of dna around 1.7 turns. The histone-octamer consists of two copies of each of the four structural core proteins (H2A, H2B, H3 and H4). The sequence encoding for H2A histone family member B1 (H2AFB1) is 517 nucleotides long (RefSeq). Based on nucleotide length alone and inferring a consistent contribution to mass from nucleotides at the single base level and in triplet at the amino acid level, each nucleosome should be roughly 95% histone by mass. But this doesnt account for stochasticity in the mass and density due to DNA-methylation and to heterochromatin and euchromatin along the genome, respectively.

Consider DNA methylation, these features represent a non-zero contribution to total chromosomal mass that would decrease histone mass as a portion of the total and would not be considered if whole genome sequencing of a single organism was employed to answer this question at the genome level for a single sample. Now consider histone acetylation, this is another feature that represents a non-zero contribution to mass but swings the pendulum in the other direction. Beyond a local calculation at the nucleosome level this relationship would be nearly impossible to quantify.

During replication specifically, histone translation seems to be as tightly regulated as DNA replication.

Ma Y, Kanakousaki K, Buttitta L. How the cell cycle impacts chromatin architecture and influences cell fate. Front Genet. 2015;6:19. Published 2015 Feb 3. doi:10.3389/fgene.2015.00019 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4315090/

nature has a great open source overview related this topic.

https://www.nature.com/scitable/topicpage/chromatin-remodeling-in-eukaryotes-1082

$\endgroup$
  • $\begingroup$ Does the 95% figure account for presence of variable-length linker DNA between the 147 bp canonical nucleosome cores? If not then this would reduce the 95%, but I think your point would still stand. $\endgroup$ – Jonathan Moore May 1 at 10:27
  • $\begingroup$ It did not, it was a rough estimate using nucleotide sequence length as a metric of weight. Using the lengths of the sequences that encode the four core histone proteins H2A, H2B , H3, H4 in relation to the length of genomic sequence wrapped around the complex the number is actually closer to 98 % `>(2*(2223 +437+917+1077))/((2*(2223+437+917+1077)) + 147)) >.9844527' H3 H4 (ncbi.nlm.nih.gov/nuccore/NM_003517.2) (ncbi.nlm.nih.gov/nuccore/NM_003528.2) $\endgroup$ – Drew J-H May 1 at 10:50
  • $\begingroup$ Something like 20% of the DNA is in linkers between the segments wrapped around the histones (here's a reference from plants plantphysiol.org/content/168/4/1406), so this would take the 98% down to 79%. $\endgroup$ – Jonathan Moore May 1 at 11:35
  • $\begingroup$ This furthers my point about the quantification only being possible at the nucleosome level. The linker segment is variable across the genome and represents another feature that prevents global quantification beyond the local nucleosome. Using your estimate of the linker region also incorporates a standard deviation associated with that estimate in nearly equivalent magnitude to the quantified region. Its a broader estimate of global proportion but greater uncertainty is associated with the estimate $\endgroup$ – Drew J-H May 1 at 12:01
  • 1
    $\begingroup$ No problem @schokakola. Happy to help. It's a pretty deep rabbit hole with implications for potential experiments. If you find the answer helpful and sufficient, you can accept it and upvote it. Why are you insterested in histone mass? Or are you more interested in potential changes to histone mass during mitosis? $\endgroup$ – Drew J-H May 1 at 21:49
2
$\begingroup$

I am reinforcing the statement made by the previous answer. Yes, histones are the most abundant proteins in the chromosome (Ohta et al. 2010); they can contribute up to ~38% of the chromosome mass. Other abundant chromosomal proteins include toposiomerase-IIα and condensin-I (SMC proteins). Apparently many non-chromosomal proteins i.e. proteins that are not a structural part of chromosome but bind non-specifically to the chromosome, can contribute up to 12% of the total protein mass. Of the purely chromosomal proteins (68%), histones form ~48%.

enter image description here (D) The 28 classes of proteins found in chromosomes.
(F) Estimated percentages of total chromosomal protein mass in the major classes of proteins.

From: Ohta et al. (2010)

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.