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Limpets prepare for a hotter climate


Changing climates have the potential to wreck havoc on living things, which are often adapted to very specific local temperatures.  These changes can alter the structure and, therefore, the function of the tens of thousands of proteins that keep cells and their owners alive.  Yet, the presence of living things in extreme environments ranging from the freezing waters of Antarctica to the boiling hot springs of Yellowstone attest to the evolutionary adaptability of proteins.  An interesting place to study this adaptive process, and the possible effects of climate change, is the complex intertidal zone along marine coasts.

George Somero of Stanford University has used many types of intertidal organisms as model systems for examining how protein structure and function adapt to environmental temperature.  In his latest paper, just published in the Journal of Experimental Biology, he has compared the enzyme cytosolic malate dehydrogenase (cMDH) in six species of limpets (the genus Lottia) along the coast of California.  These mollusk species (a group of marine snails) live at different latitudes and different zones of the intertidal, meaning that they are covered by water and baked by the sun for different amounts of time each day.  Limpets living in the upper intertidal in lower latitudes would get the most sun – so they experience the warmest body temperatures.

Lottia digitalis - the ribbed limpet
Lottia digitalis – the ribbed limpet

Somero already knew that cMDH adapted to thermal conditions from work he had done in other species.  The ability of this enzyme to run its chemical reactions changed in tune with the body temperature of these species.  Because the three-dimensional structure of cMDH was well known it was also possible to map the location of amino acid variations between species and get an idea of how they produced altered protein function.  Somero hypothesized that the six closely related limpet species differing in physiological temperature would contain variations in cMDH amino acid sequence that would provide insights into how cMDH, and proteins in general, adapt to changing environmental temperature.  He was not dissapointed.

By grinding up the muscular foot from the six species Somero and his co-author Yunwei Dong were able to collect a crude cellular extract that contained cMDH.  They found that the enzymatic activity and thermal stability of cMDH was adapted to the environmental temperature of each species, confirming their hypothesis.  When they cloned and sequenced the cMDH genes from all six species and determined their amino acid sequences, they found 24 variable residues.  What is really interesting is that the amount of variation between any two amino acid sequences did not correlate with differences in protein function or thermal stability.  The amount of structural variation did not predict how different the proteins would behave.

Two species in this study are of particular interest.  The northern and cold adapted L. digitalis and more southerly warm adapted L. austrodigitalis are so closely related that they are often difficult to tell apart from their physical appearance.  But their respective cMDH proteins are the most divergent in function and thermal stability of the six in the study, matching their divergent latitudes.  And here is the kicker – they only differ by one amino acid.  When you map that one residue onto a computer generated 3D structure for cMDH it falls in an important substrate binding site.  

The 3D structures of two limpet cMDH proteins.  The one amino acid difference between the two is at position 291, shown on the left.
The 3D structures of cold (left) and warm (right) adapted limpet cMDH proteins. The one amino acid difference between the two species is at position 291, shown on the right side of each molecule.

The amino acid variation in the warm adapted L. austrodigitalis provides greater hydrogen bonding and reduced flexibility – this means that this version of the protein can more stably stick to the molecules that it acts on.  A neat trick when it needs to grab another molecule at elevated temperatures.

How does this research relate to global warming?   Between the 1970’s and 1990’s the range of the southerly L. austrodigitalis expanded northward into Monterey Bay at the same time that waters in the Bay increased in temperature.  The southern range of L. digitalis moved northward during this same period.  These species are shifting northward as waters warm since their proteins (cMDH and presumably others) are finely tuned to a relatively narrow range of temperature extremes.  But this paper also suggests that proteins have the potential to adapt to warming environments.  In this case only one amino acid substitution is needed to dramatically change the ability of this protein to function at higher temperatures.

Will the powerful adaptive ability of proteins allow life on Earth to adapt to our warming planet?  Unfortunately that grand experiment is underway.  Perhaps bittersweet for the comparative physiologist.

Y. Dong, G. N. Somero (2009). Temperature adaptation of cytosolic malate dehydrogenases of limpets (genus Lottia): differences in stability and function due to minor changes in sequence correlate with biogeographic and vertical distributions Journal of Experimental Biology, 212 (2), 169-177 DOI: 10.1242/jeb.024505

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