Many do not place ‘bacteria’ and ‘memory’ in the same sentence. Normal human perception does not connect the two concepts. However, Mother Nature seems to have a more profound perception. The past 50 years or so of scientific investigation has shown how our uniqueness as humans is actually commonplace across all forms of life on Earth. Case in point, how closely associated molecular memory is between bacteria and human.
Bacteria use adaptation to signals as memory
Swimming bacteria do not move randomly in their environment. This behavior would be futile and counterproductive. Instead, bacteria are constantly monitoring their environment in search of food and poisons. Moving towards the former and away from the latter. This observation was first published in the late 19th century. Bacteria, like the famous and infamous E. coli, use molecular antennae to receive these important ‘signals’ as the basis in the decision of which direction to swim. What if the bacteria find a great place to reside with lots of food but still need to receive signals to ensure they remain there? The antennae have sections that can be modified easily and reversibly. These modifications, in the form of methylation, alter the sensitivity of the antenna protein to subsequent signals. Methylation allows these antennae not to receive the number of absolute signals but relative signals. In other words, the antenna protein through fine-tuned methylation detects changes in the number of signals now versus some time in the past. This is the basis of molecular memory.
These antennae are proteins called methyl-accepting chemotaxis proteins, or MCPs. MCPs accept methyl groups from the essential cofactor S-adenosylmethionine (aka SAM or AdoMet). AdoMet is essential to both prokaryotes and eukaryotes like humans. The methyl groups are added by a protein called CheR (pronounced ‘key R’) which transfers the methyl from AdoMet to very specific amino acid side groups of glutamate. The process, called O-methylation adds the methyl group to the single-bonded oxygen on the carboxyl.
The length of a bacterium’s molecular memory is very short in comparison to how we perceive memory at only a few seconds. But, to bacteria it is long enough to successfully navigate the environment with similar precision when concentrations of food or poison vary (up to several orders of magnitude, or ~1000x).
Does the basis of molecular memory in humans mimic bacteria?
Eukaryotes, including humans, use a very similar mechanism in signal transduction to bacteria. Phosphorylation (transferring a phosphate group from ATP or GTP to a protein amino acid) is the basis of all signal transduction and cell regulation. Bacteria use histidine kinases and response regulators, as do plants to some degree. However, the majority of regulation through signal transduction in eukaryotes is through two types of proteins, RAS proteins and the heterotrimeric G-proteins. G-proteins interact with membrane receptors that regulate their activity. What determines which surface receptors G-proteins interact with? Isoprenylcysteine methyltransferase, or ICMT, is one of two methyltransferases that regulate signal transduction activity. ICMT is a membrane protein that uses AdoMet to add methyl groups to isoprenylcysteine, a post-translationally modified cysteine residue on both heterotrimeric and RAS-related G proteins. Methylation regulates which receptors the G-proteins interact with, thus playing a major role in connecting the initial signal to downstream regulatory pathways. The carboxyl methylation essentially modulates G-protein signalling globally.
G-protein carboxyl methylation is regulated by GPCR signaling and, as seen above, GPCR signaling is regulated by G-protein carboxyl methylation. This feedback/feed forward loop could be seen as a form of molecular memory stored in methylation patterns. Within the brain, ICMT activity is almost exclusively found in the region controlling coordination of movement. Thus, methylation could be used to modulate certain neuronal signaling pathways which result in learned patterns of sensory-motor skills.
The only other major methyltransferase is from a protein known as PPMT. PPMT interacts with a major enzyme in signal termination, the protein phosphatase PP2A. PPMT adds methyl groups to the backbone carboxyl of a specific leucine in PP2A. This carboxyl methylation helps determine which B subunit PP2A interacts with and where in the cell PP2A can be found. PPMT structurally resembles CheR in bacterial memory. Moreover, the enzyme that removes the methyl group from PP2A, PME, structurally resembles the bacterial enzyme that removes methyls from MCPs, CheB.
PP2A is one of the major regulators of pathway coordination to maintain synaptic plasticity in the brain. Interestingly, methylation defects and PP2A-PME complexes are suggested to play a role in the cause of Alzheimer’s Disease and memory loss. Methylation defects leading to defective phosphatase activity of PP2A leads to accumulation of a phosphorylated subunit of the structural protein microtubule. In this phosphorylated form, the filaments used to keep axons structurally sound collapse and lead to loss of normal synapses. Therefore, molecular memory in the form of methylation plays a vital role in promoting normal brain activity and its disruption can ultimately lead to dementia.
Chicken, meet egg. Egg, meet chicken.
So, from bacteria to human, carboxyl methylation is necessary for memory. Did these pathways evolve individually in parallel, or did the memory we have today originate in the predominant lifeforms found within us?
Li and Stock. (2009) Biol. Chem. 390: 1067-1096. DOI 10.1515/BC.2009.133