Perpetual Ltd has the Australian Prudential Regulation Authority to thank for the growth in its corporate trusts business over the past year.Perpetual Corporate Trust is the biggest provider of trustee services to the Australian securitisation market, holding A$427 billion of securitisation funds under administration.With issuance of residential mortgage-backed securities falling off this year, Perpetual has had the benefit of growth in so-called repo eligible RMBS - securities that are placed in a Reserve Bank committed liquidity facility to allow banks to meet APRA's liquidity target.Over the six months to June Perpetual's securitisation funds under administration increased by 3.4 per cent to $427.5 billion.Bank and non-bank RMBS fell during that period, while there was a small increase in asset-backed securities (mostly auto loan securitisations) and covered bonds.The big increase was in repo RMBS - up from $203 billion to $212 billion.Perpetual Corporate Trust also has a funds services business providing trustee and custody services to the funds management industry. Funds under administration in the fund services business rose from $186 million to $193 million over the six months to June.Perpetual Corporate Trust contributed pre-tax profit of $34.2 million to group earnings for the year to June - down nine per cent from $37.5 million in 2014/15.One reason for the lower profit was that repo RMBS earns a lower margin than other types of securitised assets.