Table 1.

List of various human canonical and non-canonical PAPs.

Name of the PAP	Localisation	Functional significance	Reference
Canonical PAP
PAPα       PAP I       PAP II       PAP III       PAP IV       PAP V       PAP VI	Nuclear	PAP II is the most predominant PAP; involves in general 3′-end processing of all nuclear nascent pre-mRNAs (PAP I and IV – function not clear, likely similar to PAP II) PAP V, III, VI are truncated inactive form, Do not encode functional proteins	[16,18,94,102,114]
PAP β (PAPT)	Nuclear/Cytoplasmic	Testes specific – spermatogenesis	[12,15]
PAP γ/neoPAP	Nuclear	Tumourigenesis, monoadenylation activity towards small RNA	[13,17,112]
Non-canonical PAP
PAPD1 (hmtPAP)	Mitochondrial	Mitochondrial mRNA stabilisation, histone mRNA degradation, stop codon regeneration	[118,120–123]
PAPD4 (hGld2)	Nuclear/cytoplasmic	Cytoplasmic mRNA polyadenylation, miRNA stabilisation	[122,125,126,128] For review [127,129]
PAPD5	Nuclear	Aberrant rRNA degradation, histone degradation, processing of snoRNAs, various other RNA targets	[21,139–141]
POLS (PAPD7)	Nuclear	Not clearly defined, likely redundant to PAPD5	[21,140]
ZCCHC6 (TUT 4)	Nuclear	Similar to ZCCHC11; regulate Let 7 biogenesis	[147,157]
ZCCHC11 (TUT 6)	Nuclear	miRNA regulation (let7, mi26a and others), histone mRNA degradation	[124,143,147,157]
Star-PAP (RBM 21, TUT1)	Nuclear	Oxidative stress response, DNA damage induced apoptosis and various other cellular functions	[19,20,23,24]