Prolactin Orthologue

Molecular review of prolactin with orthologue comparisons of Homo Sapien, Danio rerio, and Mus musculus.

Andrea Duchow

I. Introduction to the Molecule:

Prolactin is a hormone (released by the anterior pituitary gland) that is most commonly known and named for its ability to stimulate lactation in female mammals, but additionally, serves several homeostatic functions and its involvement has been recorded in 300 biological processes (Freeman et. al 2000). Immune responses are enhanced by elevated levels of prolactin and even behavioral effect. For example, expression in rats has demonstrated parental behavior, such as nest building. Additionally, it is important to consider the vital function prolactin orthologues play

in the osmoregulation of fish (Sakamoto and McCormick, 2006).

In freshwater, a dilute environment, fish must have the ability to excrete water and either retain or uptake ions against the concentration gradient. Prolactin allows them to accomplish this by reducing water and ion permeability and enhancing the uptake of nearby ions. The contributions of many scientists went into the discovery of human prolactin as an individual pituitary hormone, but one of the most notable contributions was done in the early 1930s by Dr. Oscar Riddle, who developed the crucial pigeon crop sac bioassay that played a role in naming prolactin because the sac thickened and lactated after injections of pituitary extracts (Smith, 2004). In the earliest studies of prolactin, pigeons and doves were used due to the strong relationship Dr. Riddle shared with avian physiologist, Dr. Charles Whitman. This assay, most importantly, allowed prolactin to be identified as an independent pituitary hormone, allowed for the isolation of it from other hormones, and set the foundation for future studies of the physiological effects of prolactin (Smith, 2004). Prior to the 1970s, the existence of human prolactin was doubted to exist, and although it had been identified in various other mammals, endocrinologists believed that human growth hormone (HGH) was

the closest hormone that could be found in humans (Friensen, 1995). It wasn’t until the development of radioimmunoassays, in the 1970’s, that prolactin was identified in humans. It was first sequenced in 1981 by Nancy E. Cooke and colleagues.
II. The Structure:
This protein is typically 227 amino acids long in humans; however, only 198 of those amino acids serve a “prolactin-like” function, which is responsible for the binding and signaling of prolactin hormone (Figure 1). The functional region’s primary, secondary, and tertiary structure can be seen in figure 2. Binding often involves helices 1 and 4, but may also involve helices 1 and 3.
CLUSTAL W (1.83) multiple sequence alignment
humPro --MNIKGSPWKG-SLLLLLVSNLLLCQSVAPLPICPGGAARCQVTLRDLFDRAVVLSHYI 57
fishPro ---MAQGS-------RQYFAVAILMCAFVSIN----------GVGLNDLLDRASQLSDKL 40
mousPro MTMNSQGSAQKAGTLLLLLISNLLFCQNVQPLPICS--AGDCQTSLRELFDRVVILSHYI 58
:** : :*:* * . *.:*:**. **. :

humPro HNLSSEMFSEFDKRYTHGRGFITKAINSCHTSSLATPEDKEQAQQMNQKDFLSLIVSILR 117
fishPro HYLSTSLTNDLDSHFPPIGRGMMPRPSLCHTSSLQIPNDKDQAMKVPEDELLSLARSLLL 100
mousPro HTLYTDMFIEFDKQYVQDREFMVKVINDCPTSSLATPEDKEQALKVPPEVLLNLILSLVQ 118
* * :.: ::*.:: : .

* **** *:**:** :: . :*.* *::

humPro SWNEPLYHLVTEVRGMQEAP-EAILSKAVEIEEQTKRLLEGMELIVSQVHPETKENEIYP 176

fishPro AWSDPLALLSSEASSLAHPERNTINSKTKELQDNINSLGAGLEHVVHKMGSSSDNLSTLP 160

mousPro SSSDPLFQLITGVGGIQEAP-EYILSRAKEIEEQNKQLLEGVEKIISQAYPEAKGNGIYF 177

: .:** * : . .: .. : * *:: *:::: : * *:* :: : ..:.

humPro VWSGLPSLQMADEESRLSAYYNLLHCLRRDSHKIDNYLKLLKCRIIHNNNC-- 227

fishPro FNGNN---LGQDKTSRLVNFHFLLSCFRRDSHKIDSFLKVLRCRAAKKRPDMC 210

mousPro VWSQLPSLQGVDEESKILSLRNTIRCLRRHSHKVDNFLKVLRCQIAHQNNC-- 228

. . *: *:: : *:**.***:*.:**:*:*: ::.

Figure 1. A multiple alignment comparison of orthologues prolactin from a human, zebrafish, and mouse generated using CLUSTAL W (1.83) multiple sequence alignment (Larkin MA et. al 2007). The asterisk denotes identical residue, colons denote similar amino acids, and periods denote slightly similar amino acid residues in all three sequences. The blue box represents a domain predicted to be responsible to the prolactin binding and signaling site on the protein in humans (Cooke et al. 1981). A similar “prolactin-like” region is denoted by a red box on the mouse prolactin sequence (Hagaya et. al 1986). The yellow highlight denotes signal peptides, located before the amino acids in the “prolactin-like” regions of the human and mouse, which are used to specify

different cellular placement and are later cleaved. The underline denotes the “growth hormone like” region in the zebrafish, which is part of the prolactin hormone family (Herzog et. al 2003).
Homo sapien Prolactin Structure

Figure 2. This image contains the primary and tertiary structure. It is important to note that 50% of this sequence contains 4 long alpha-helices, while the rest of it loops loosely. The black lines denote disulfide bonds.
III. The Biosynthesis
The pathway and hormones involved in the stimulation and release of prolactin can be seen in figure 2. It is important to note that special cells, called lactotrophs, compromise 20-50% of an organism’s anterior pituitary gland that are responsible for the secretion of prolacti