The anterior pituitary is a part of the endocrine system; it is composed of several cell types which create various cell populations that secrete specific hormones (Hiraoka et. al.
1991). Prolactin and growth hormone are two hormones that the anterior pituitary secretes. These hormones are closely related and are form the same family of peptide hormones (Corbacho et. al. 2002). Although prolactin and somatotropin are hard to distinguish from one another, prolactin has a broader spectrum of biological activities in vertebrates (Li 1973). It plays a role in mammary-gland development, induces brood-patch development in a variety of species, increases weight in tadpoles, and regulates osmotic pressure in fish (Li 1973).
By testing the relatedness of the hormones, we were able to determine whether paralogous hormones or orthologous hormones shared a higher percentage of sequence identity.
Methods
Animals Used
Since we were using the information gathered from the bioinformatic databases, animals were not used to perform our experiment.
Experimental Procedure
We collected the following sequences from the NCBI Protein Database: Human Prolactin (CAA38264; Hiraoka et al. 1991) Pig prolactin (P01238; Scultuz-Aellan et. al. 1989), Human Somatotropin (NP_000506; Bailey et. al. 2010), and Fish Prolactin (NP_852102.2; Wang et. al. 2010). All sequences were saved in their FASTA format as .text files in order to be edited with no gaps or spaces. They were then analyzed for sequence identity. To analyze for sequence identity, we performed binary alignment using L-ALIGN (Huang and Miller 1991). A
multiple sequence alignment using CLUSTAL W at EBI was also performed (Higgins et al. 1994). The results were recorded and the sequences saved for further assessment.
RESULTS
We tested whether orthologous proteins shared a different amount of sequence identity than paralogous proteins by comparing an alignment human and pig prolactin (78.6%) with an alignment of human prolactin and human growth hormone (23.9%). We also compared an alignment of human and fish prolactin (32.6%) with an alignment of human prolactin and human growth hormone. We also created a multiple sequence alignment to show overall conservation patterns in paralogous and paralogous pituitary hormones (Fig 1).
humprl -------MKGS--LLLLLVSNLLLCQSVAPLPICPGGAARCQVTLRDLFDRAVVLSHYIH 51
pigprl MDNRGSSQKGSLLLLLLLVSNLFLCKSVASLPICPSGAVNCQVSLRDLFDRAVILSHYIH 60
fishprl ------MAQGS---RQYFAVAILMCAFVSIN----------GVGLNDLLDRASQLSDKLH 41
humgh ------MATGS-------RTSLLLAFGLLCLPWLQEGSAFPTIPLSRLFDNAMLRAHRLH 47
** :::. : : * *:*.* :. :*
humprl NLSSEMFSEFDKRY-THGRG---FITKAINSCHTSSLATPEDKEQAQQMNQKDFLSLIVS 107
pigprl NLSSEMFNEFDKRY-AQGRG---FITKAINSCHTSSLSTPEDKEQAQQIHHEVLLNLILR 116
fishprl YLSTSLTNDLDSHFPPIGRG---MMPRPS-LCHTSSLQIPNDKDQAMKVPEDELLSLARS 97
humgh QLAFDTYQEFEEAYIPKEQKYSFLQNPQTSLCFSESIPTPSNREETQQKSNLELLRISLL 107
*: . .:::. : . : : *.:.*: *.::::: : . :* :
humprl ILRSWNEPLYHLVTEVRGMQEAP-EAILSKAVEIEEQTKRLLEGMELIVSQVHPETKENE 166
pigprl VLRSWNDPLYHLVTEVRGMQEAP-DAILSRAIEIEEQNKRLLEGMEKIVGQVHPGIKENE 175
fishprl LLLAWSDPLALLSSEASSLAHPERNTINSKTKELQDNINSLGAGLEHVVHKMGSSSDNLS 157
humgh LIQSWLEPVQFLRSVFANSLVYG-----ASDSNVYDLLKDLEEGIQTLMGRLEDGSPRTG 162
:: :* :*: * : . : :: : : * *:: :: :: .
humprl IYPVWSGLPSLQMADEESRLSAYYNLLHCLRRDSHKIDNYLKLLKCRIIHNNN--C 220
pigprl VYSVWSGLPSLQMADEDTRLFAFYNLLHCLRRDSHKIDNYLKLLKCRIIYDSN--C 229
fishprl TLPFNG---NNLGQDKTSRLVNFHFLLSCFRRDSHKIDSFLKVLRCRAAKKRPDMC 210
humgh QIFKQTYSKFDTNSHNDDALLKNYGLLYCFRKDMDKVETFLRIVQCRSVEGSCGF- 217
.: * : ** *:*:* .*::.:*::::**
Discussion
Evolution has not only changed the physical appearance of organisms, but it has had a tremendous affect on the production and function of certain hormones.
We found that orthologous protein hormones in our experiment which are: human, fish, and pig prolactin, had a higher related identity than the paralogous proteins, human prolactin and human somatotropin. We also noticed that the related identity of the prolactin in hormone in mammals was higher than the related identity of the prolactin hormone between mammals and fish.
It is well known that prolactin in mammals help promote milk production (Harvey et. al. 1995). In some fish the function of prolactin seems to be different from the function of prolactin in mammals. In fresh-water fish, prolactin plays an important role in osmoregulation; it is even sometimes given the alternate term of paralactin (Harvey et. al. 1995). This difference may have stemmed from the evolution of fish to mammal. There is speculation that there were multiple gene duplications from the ancestral somatolactin hormone in fish to the prolactin hormone in modern day fish and mammals (Harvey et. al. 1995). Another cause for this difference between fish and mammal prolactin may be the environment in which these two organisms live. Fish in general, with the exception of whales and dolphins which are mammals, do not feed milk to their young. The prolactin gene may have coded for milk production at a higher percentage than it did for osmotic pressure regulation, but at some point during the evolutionary change that particular code was turned off and the osmoregulating code turned on. Another reason for the difference could be a product of natural selection. Fish that has prolactin that managed osmoregulation survived and reproduced better in their aquatic environment, causing a decrease in fish with the milk producing prolactin until there were none left.
In paralogous proteins, the differences might stem from gene duplication. Growth hormone and prolactin almost act as a team, while prolactin stimulates milk production;
growth hormone stimulates the growth of mammary glands in the breast (Li 1973). The differences may be because of the function; one acts as a hormone that increases gland size, while the other acts as a hormone that increases gland production.
In conclusion, we found that protein hormones that are orthologous have a higher related sequence identity than those that are paralogous.
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